Environment, genes, and experience: Lessons from behavior genetics

Journal of Physiology - Paris 104 (2010) 243–252

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Journal of Physiology - Paris journal homepage: www.elsevier.com/locate/jphysparis

Review Paper

Environment, genes, and experience: Lessons from behavior genetics Philipp I. Barsky ⇑ Psychological Institute of Russian Academy of Education, Kashirskoe shosse 80-2-676, Moscow 115569, Russian Federation

a r t i c l e

i n f o

Keywords: Behavior genetics Nonshared environment Heritability Gene–environment interplay Psychoanalysis

a b s t r a c t The article reviews the theoretical analysis of the problems inherent in studying the environment within behavior genetics across several periods in the development of environmental studies in behavior genetics and proposes some possible alternatives to traditional approaches to studying the environment in behavior genetics. The first period (from the end of the 1920s to the end of the 1970s), when the environment was not actually studied, is called pre-environmental; during this time, the basic principles and theoretical models of understanding environmental effects in behavior genetics were developed. The second period is characterized by the development of studies on environmental influences within the traditional behavior genetics paradigm; several approaches to studying the environment emerged in behavior genetics during this period, from the beginning of the 1980s until today. At the present time, the field is undergoing paradigmatic changes, concerned with methodology, theory, and mathematical models of genotype–environment interplay; this might be the beginning of a third period of development of environmental studies in behavior genetics. In another part, the methodological problems related to environmental studies in behavior genetics are discussed. Although the methodology used in differential psychology is applicable for assessment of differences between individuals, it is insufficient to explain the sources of these differences. In addition, we stress that psychoanalytic studies of twins and their experiences, initiated in the 1930s and continued episodically until the 1980s, could bring an interesting methodology and contribute to the explanation of puzzling findings from environmental studies of behavior genetics. Finally, we will conclude with implications from the results of environmental studies in behavior genetics, including methodological issues. Ó 2010 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Behavior genetics in the pre-environmental era . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Two components of environmental variance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Objective environment and effective environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The development of environmental studies in behavior genetics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Environment as a phenotype: Nature of nurture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.1. Self-reports on family environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.2. Life events . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.3. Relationships between twins and siblings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1.4. Objectively observed environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Possible explanations of correlation between genes and environmental phenotypes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. The study of ‘‘nonshared environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Nonshared environment: the observation and the hypothesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Results of nonshared environment studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. The perspectives of nonshared environment research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. The studies of environment as a mediating context for development. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.1. The findings on shared environmental influences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.2. Environmental differences and heritability. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Tel.: +7 916 8138422. E-mail address: [email protected] 0928-4257/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jphysparis.2010.08.002

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3.4.3. Measured shared environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4.4. The role of observational methods and experiments: Implications for the research of the environment . . . . . . . . . . . . . . . . . . Outside behavior genetics: psychological and psychoanalytic studies of twins, their environment and experience . . . . . . . . . . . . . . . . . . . . . . Beyond the genes and environment: behavior genetics and the dynamics of human experience . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Since the beginning of the 20th century, behavior geneticists have studied the relative contribution of genetic and environmental factors to individual differences in psychological and psychophysiological traits. Many studies have been conducted on the heritability of different psychological and psychophysiological traits, but empirical research of environmental influences started in behavior genetics only two decades ago — although theoretical proposals were made much earlier (for a review, see Anastasi, 1958). The primary goal of behavior genetics for a long period of time was to demonstrate the impact of genetic factors on individual differences in behavior; this goal in general has succeeded to the present moment (Bouchard and Loehlin, 2001; Plomin and Colledge, 2001). Researchers determined that almost all traits under study revealed at least a moderate degree of heritability (Bouchard, 2004; Turkheimer, 2000). However, starting from the early 1980s, the question of how the environment influences the development became progressively one of the main interest in behavior genetics. It was clear that it was impossible to study the genetic influence on a trait without studying the concrete environmental context of its development, but the current methods of behavior genetics do not actually allow for this. We argue here that behavior genetics is currently undergoing paradigmatic changes that are, in part, stimulated by the results of studies on environmental influences. Moreover, we would like to stress that the methodology of statistical analyses, used in behavior genetics for the last decades, is now reaching a point where its application precludes from solving fundamental problems raised in behavior genetic studies of the environment. It is noteworthy that initially, environment was understood in behavior genetics as a totality of all non-genetic conditions necessary for the development of a physiological or psychological trait under study. Besides genotype, environment was considered as the other main factor leading to all the possible varieties of individual differences, called phenotype. This understanding of environment is broader than what is usually called ‘‘environmental” in psychology; it includes not only socialization conditions in family and community, but also many other types of non-genetic influences, such as prenatal influences, nutrition, or illnesses (Plomin and Caspi, 1999). Although the term environment is prevalent in behavior genetics, no general theory of environmental influence has been currently developed (Schaffner, 2006). The long dominating idea of two-factor determination—that it is reasonable and possible to separate genetic and environmental sources of development—has been widely criticized, not only by psychologists (Hoffman, 1991; Leontiev, 2005; Maccoby, 2000; Wachs, 1983), but also by geneticists (Gottlieb, 1995; Lickliter and Honeycutt, 2003). Some authors propose alternative concepts, such as ‘‘ecology” or ‘‘contexts” of development, instead of ‘‘environment” (Bronfenbrenner, 1979; Bronfenbrenner and Ceci, 1994; Lerner, 1995). Nevertheless, we will stick to the term environment in this article, following the age-old tradition of behavior genetics. First, the article reviews the different periods in the development of environmental studies in behavior genetics. The first

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period (from the end of the 1920s to the end of the 1970s), when the environment was not actually studied, is called pre-environmental. During this time, researchers paid more attention to the contribution of genetic factors to individual differences in psychological traits, including the concept of heritability. The next period of development of environmental studies began with publications by Plomin and collaborators (Rowe and Plomin, 1981; Plomin and Daniels, 1987), which were stimulated by his hypothesis of the individual (nonshared) character of environmental influence on development. The beginning of this period can be marked by the first studies of genetic influence on perceptions of the family environment (Rowe, 1981). The preliminary results of this line of research are summed up in publications from the project ‘‘Nonshared Environment and Adolescent Development” (NEAD: Reiss et al., 2000) and in the Turkheimer and Waldron critical review (2000). At the present time is beginning a new period involving a rethinking of the models of environmental influence with a search for new paradigms in behavior genetics (Reiss et al., 2000; Rutter, 2006). Each of these three periods can be characterized by its own special conceptualization of environment and strategies of empirical research.

2. Behavior genetics in the pre-environmental era We named the first and the longest period of studies in behavior genetics pre-environmental because the environment was not actually studied at that time, save rare exceptions. The main aim of researchers was to obtain the heritability coefficients for the most important psychological characteristics (such as intelligence, personality, and psychopathology). Despite the fact that environment was not studied (we are talking here only about the area of behavior genetics, and not the other conceptually different fields of psychology, as will be shown further in the article), the main conceptual assumptions that determined the understanding of environment in behavioral genetic research during the most recent decades were formulated at that time.

2.1. Two components of environmental variance The most primitive understanding of environment was, as mentioned, the ‘‘sum of all non-genetic influences”. This summary effect of environment can be evaluated statistically as a residual term (e)—percentage of variance that is not explained by genetic factors (heritability)—i.e., e = 1 h2. This is how environment was depicted in behavior genetics before the 1970s (for critics, see Wachs, 1983). However, with the development of more advanced methods of mathematical analysis of data, especially structural equation modeling (or model-fitting), behavior genetics started approaching a more sophisticated analysis of environmental effects. The progress in theory of quantitative genetics (Jinks and Fulker, 1970) and in statistical methods (Rao et al., 1974; Jöreskog, 1973, 1978) led to the invention of an idea that non-genetic variance can be further partitioned into two components: within-family (also called nonshared, unique, or

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specific) and between-family (also called shared, common, or family) environmental variance.1 Nonshared environmental variance results from all non-genetic influences that contribute to differences between family members as well as from unreliability of measurement and other non-systematic factors that create noise in data. The single most direct measure of the impact of nonshared environmental factors is the variation within pairs of monozygotic twins reared together (Neale and Cardon, 1992). Shared environmental variance results from all non-genetic influences that contribute to resemblance between family members. As the interpretation of these two indices of environmental influences has always been problematic—since their invention until present time—we consider clarification necessary at this point. First of all, the nature of the two kinds of environmental influences lies in the assumptions made in biometrical genetics about how the environment operates (Mather and Jinks, 1971). It must be understood that evaluations of within- and between-family environmental variance components were traditionally made on the basis of measurement of probands’ phenotype only, without measuring any concrete sources of environmental influence in the real world (in the same way that heritability could be calculated without knowing concrete genes). To the extent that they are derived from the heritability coefficient, they are prone to the same weaknesses and limitations as heritability (Erdle, 1990). 2.2. Objective environment and effective environment To explain the essence of the problem of the interpretation of nonshared and shared environmental variance components, we will refer to the notions of effective environment and objective environment. This distinction was first proposed by Goldsmith (1993) and further developed by Turkheimer and Waldron (2000). In his work, Goldsmith emphasized that researchers should explicitly separate environmental causes and their effects on variation among people. The processes and the effects are different: an objectively ‘‘shared” event may have a ‘‘nonshared” effect, and vice versa. As it was put by Turkheimer and Waldron: ‘‘Objective environments refer to environmental events as they might be observed by a researcher, as opposed to how they affect family members. Therefore, the question of whether objective environments are shared or nonshared refers only to whether or not they constitute the environment of more than one sibling in the family, regardless of whether their effects serve to make siblings more alike or more different. Many traditional between-family environmental variables, like socioeconomic status and marital discord, are objectively shared in this sense. Objectively nonshared events are those, like peer relationships and birth order, that constitute the environment of only one sibling, again regardless of whether they work to make siblings alike or different. Effective environments are defined by the outcomes they produce. The estimate of shared environmental variation that results from biometric studies refers to the effect of environments in creating sibling resemblance, regardless of whether the objective environments were shared or nonshared. Thus, if an objectively shared environmental variable results in nonshared effects, the effective contribution of the objectively shared event is included with the nonshared rather then the shared component of variance” (Turkheimer and Waldron, 2000). 1 The similar terms and mathematical formula for discerning between ‘‘bringing together” and ‘‘bringing apart” kinds of environment were first suggested in the 1930’s by Russian scientist M. V. Ignatiev; however, his works have not obtained recognition in Western psychology (see Ignatiev, 1934, pp. 18-33; Ravich-Scherbo et al., 1999, pp. 43-44).

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Thus, shared and nonshared ‘‘environments” in behavioral genetic studies of pre-environmental period must be understood as only evaluations of the results of environmental influences and not the evaluations of the influences themselves (Rutter, 2006). No way exists to derive real, objective sources of environmental variance post-factum, using data from subjects’ phenotypes only (Reiss et al., 2000). Hence, the common practice of explaining the meaning of shared and nonshared environmental variance without measuring the objective environment (such as ‘‘shared environmental factors represent common family conditions for siblings such as socio-economic status” or ‘‘nonshared environmental factors represent siblings’ different friends and different teachers in school”) must be considered as a mere speculation.2 Unfortunately, such practice of taking for granted the meanings of environmental variance components have led some scientists to rather severe distortions of results from behavioral genetic studies as well as to dubious social policy recommendations. By the end of the 1970s the methodology of twin and family studies had been perfectly developed, advanced statistical algorithms of quantitative genetic analysis had been invented (Martin and Eaves, 1977), and an impressive amount of data from twin studies had been collected (Loehlin and Nichols, 1976). It became evident that the contribution of environmental factors for most of the traits under study was not less, but in many cases, more than the contribution of genetic factors. Yet, at the same time, these environmental factors have remained anonymous. The dissatisfaction of researchers with such non-interpretable results, together with increasing criticism from developmental psychology, stimulated the development of new approaches in behavior genetics that aimed to study the effect of measured environmental factors while controlling for genetic influences. By the beginning of the 1980s, the next period could be marked out: the period of development of environmental studies in behavior genetics. The next section will review the most important directions of research in this period.

3. The development of environmental studies in behavior genetics During the period of the establishment and development of environmental studies within behavior genetics, three significant groups of studies can be discerned. These three approaches differ according to the objects of interest as well as by their aims and methodology. The first approach is based on the assumption that we may apply the rules of biometrical analysis to any kind of individual characteristics, including one’s environment (both subjectively perceived and objectively observed) (Plomin and Bergeman, 1991). The environment is understood here to be primarily the psychological surroundings of a person. By correlating monozygotic (MZ) and dizygotic (DZ) twins’ or other relatives’ self-reports on their environments, heritability estimates are obtained for different kinds of perceived environments that are taken as a phenotype. The results of such studies show that a significant amount of variance in environmental characteristics can be associated with genetic factors. The same pattern is revealed in observational studies, where environment is assessed objectively, although these studies are very labor intensive. The results of this line of research are usually heritability coefficients for the different characteristics of environment, which obviously could have no direct connection to genetic factors. Nevertheless, environmental characteristics show significant genetic influence. The theories of genotype– 2 Examples of this speculation can be found in many textbooks and journal articles on behavior genetics and psychology.

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environment interaction (Plomin et al., 1977; Scarr and McCartney, 1983) are being drawn for the explanation of those facts. This approach is sometimes referred as ‘‘nature of nurture” (Plomin and Bergeman, 1991). Another direction of environmental studies in behavior genetics started developing in the early 1990s: the study of specific nonshared environmental factors. According to a hypothesis put forth by Plomin and collaborators (Rowe and Plomin, 1981; Plomin and Daniels, 1987), the significant non-genetic factors in development are those that make siblings in the same family more different from each other. This hypothesis has generated a whole new area of research on siblings and twins, focusing on possible objective sources of nonshared environmental variances in different behavioral outcomes (Turkheimer and Waldron, 2000; Plomin et al., 2001; McGuire, 2001). The third group of studies, although less integrated than former two, also provides important material for understanding how environment might be studied within a behavior genetics framework. These include biometrical studies in which environmental influences are explicitly controlled via experimental manipulation (Malykh, 2004a) or indirectly by comparing samples of genetically related people living within different environmental conditions (Rowe et al., 1999; Turkheimer et al., 2003). 3.1. Environment as a phenotype: Nature of nurture The early works that investigated genetic contribution to environment as a phenotype were using the standard methodology of twin studies. As with any other phenotype—for example, personality traits—questionnaire measurements were used; however, this time the questions were asked about the different features of parent–child relationships, siblings’ interactions with each other, and other aspects of familial and extra-familial environment. In addition, much fewer studies of environment as phenotype were conducted in which observational measurements were employed. 3.1.1. Self-reports on family environment Rowe (1981) was the first to use twins’ reports on their parents’ behavior. He found that MZ twins give much more similar reports on their parents’ ‘‘warmth” than DZ twins, but the same was not found to be true for parental ‘‘control”. A second study by Rowe (1983) was conducted on twins and regular siblings, yielding similar results for the factors or parental ‘‘control” (no heritability) and parental ‘‘warmth” (moderate heritability). The authors of the Colorado Adoption Project reported that genetic factors might exist that mediate the influence of different characteristics of family environment on children’s behavioral outcomes (Plomin et al., 1985; Coon et al., 1990). The results of a longitudinal analysis on the same sample are consistent with previous reports: ‘‘emotional” (parental ‘‘warmth” and ‘‘negativity”) aspects of family interaction seem to be correlated with genetic factors, while ‘‘cognitive” aspects (parental ‘‘control”) are not (DeaterDeckard et al., 1999). The most dimensioned project aimed at discovering the role of genetic and environmental influences on perceptions of family environment was the longitudinal study of six types of families (families with MZ and DZ twins, families with regular twins, and different kinds of stepfamilies), entitled ‘‘Nonshared Environment and Adolescent Development” (NEAD: Reiss et al., 2000). The authors reported moderate heritability for 15 aggregated scales from several questionnaires on different aspects of family environment (Plomin et al., 1994). One popular strategy of studying the heritability of different environmental phenotypes is retrospective approach, which utilizes adults’ memories of their family environment in childhood. This strategy was widely used in studies of middle-aged twins

who had been reared together and apart. The results of the Swedish Adoption/Twin Study of Aging (SATSA: Plomin et al., 1988, 1989) suggested a genetic contribution to perceptions of family environment as rated from 50 years later. The authors of the Minnesota Study of Twins Reared Apart (MSTRA: Hur and Bouchard, 1995; Krueger et al., 2003) also reported that a contribution of genotype might exist for differences in retrospective reports of adult twins on their rearing environment in childhood. These results are in general consistent with those obtained by Rowe in the earlier mentioned works. 3.1.2. Life events The special group of studies of genetic influence on environmental phenotypes includes studies of life events. These also use retrospective reports given by adults using questionnaires on different kinds of events that had or had not happened in their lives. The first studies suggested moderate heritability for some life events (Plomin et al., 1990). Later it was shown that the contribution of genetic factors to the differences in retrospective reports of adults on their life events varies according to the degree of subjective control of the event: the more controllable life events yield a higher heritability than less controllable ones (Billig et al., 1996; Kendler et al., 1993). The latest studies of genetic influence on retrospective reports on life events are characterized by especially large samples. Among a sample of 3938 pairs of twins, Bolinskey et al. (2004) found that ‘‘personal” life events (those that are more affected by the person) revealed some degree of heritability, but ‘‘social” life events (those that are more affected by other people) did not. Saudino et al. (1997) found that genetic factors were important only for the reports on life events for the female part of the sample of adult twins. One recent study on 5000 twins and their siblings yielded genetic contributions to retrospective reports of such kinds of life events as marriage and having close relationships (Middeldorp et al., 2005). 3.1.3. Relationships between twins and siblings The relationships in siblings’ pairs (especially in case of twins) and their influence on the development of psychological outcomes were always an important area of investigation for behavioral geneticists due to their importance to the crucial assumptions of the twin method. If the interaction in twin pairs (for example, ‘‘contrast” of ‘‘cooperation” effects) systematically affects withinpair differences, it might significantly alter the heritability of the trait under study. The first behavioral genetic studies of the relationships between siblings and twins—namely, their perceptions of their relationships with parents, siblings, and peers—were conducted with the specially designed instrument ‘‘Sibling Inventory of Differential Experience” (SIDE: Daniels and Plomin, 1985). No significant contribution of genetic factors was reported for the data on adolescent siblings in the first study (Daniels and Plomin, 1985), but the data were subsequently reanalyzed and demonstrated heritability for some characteristics of siblings’ environment, such as perceptions of peer groups (Eaves and Carbonneau, 1998). The retrospective study using SIDE on adult MZ and DZ twins also indicated that a genetic contribution might exist to reports on some within-pair differences (Baker and Daniels, 1990). It was later found that perceived within-pair differences in siblings’ relationships and environment tend to diminish with age (Pike et al., 2000). More recently, a new instrument for assessment of the withinfamily environment in twins was developed by Carbonneau and Rutter: the ‘‘Twin Inventory of Relationships and Experiences” (TIRE: Carbonneau et al., 2001). Its usefulness was demonstrated in a study of family environment and behavioral problems among 1117 pairs of adolescent twins (Carbonneau et al., 2002).

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3.1.4. Objectively observed environment All the studies previously described herein are primarily based upon data from self-reports of children or their parents. Few behavioral genetic studies have used observational methods for the assessment of environment. Data are available on videotaped mother–child interactions (Dunn and Plomin, 1986), interactions of mothers with children and siblings (Rende et al., 1992), and interactions within different types of relatives’ dyads in the family (O’Connor et al., 1995; Reiss et al., 2000). Still, the results of studies of this kind present evidence that at least some of the environmental characteristics are correlated with genotype. 3.2. Possible explanations of correlation between genes and environmental phenotypes Plomin and Bergeman published in 1991 the first review of ‘‘nature of nurture” studies, stressing that it is not enough just to call some variable ‘‘environmental” in order to determine that we know its origins because many of the measures of environment show at least some degree of heritability—i.e., a relationship with genotype. The researchers suggested that behavioral genetic studies of environmental measures are necessary to reveal the real nature of such variables. Their approach was subsequently criticized for attributing the effects of genotype–environmental interplay to the influence of genes (Rutter, 2006). As to the behavioral genetic studies of self-reports on the environment, Plomin and Bergeman hypothesized that most questionnaire measures do not give the exact representation of environment because the perceptions are ‘‘filtered” through the respondent’s personality, which is, in turn, influenced by genes. This has been, to a certain extent, confirmed in several studies (Chipuer et al., 1993; Saudino et al., 1997; Krueger et al., 2003; Spotts et al., 2005); however, no correlations between environment and personality were found in a thorough work by Vernon et al. (1997). Undoubtedly, the individual influences his or her environment. As was proposed in the strong hypothesis formulated by Scarr and McCartney (1983), people actively construct their environments in accordance with genetically influenced predispositions, which results in an environment correlated with genotype. The authors’ main point was that genes should be considered as having a predominant role in the processes of genotype–environment interaction. However, now it is generally understood that this assertion is too sharp. The interpretation of genetic contributions to individual differences in the environment as a phenotype (self-reported or observed) is accompanied by several difficulties. As it was emphasized by Rutter, the primary methodological task is the investigation of effect of the person, not the genotype, on the environment (Rutter et al., 2001). This problem was distinctly formulated for the developmental psychology by Bell (1968) and is still far from being solved. Similar ideas have been expressed by Turkheimer, who argues that the notion of the ‘‘phenotype–environment interaction” should be employed instead of the ‘‘genotype–environment interaction” (Turkheimer and Waldron, 2000; Turkheimer, 2004). Rutter and collaborators point out that the ideas of genotype– environmental interaction and genotype–environment correlation are often misinterpreted in behavior genetics: it is assumed that if a correlation between the genotype and the environment is found, it should be attributed to genetics because genes appear to be the cause that exist primary to the environment, as long as genes are inborn and environments are not. This argument erroneously mixes the actual causes of individual differences that could have purely environmental origins (for example, availability of drugs) and non-specific genetically influenced risks (for example, the proneness to having a drug addiction) that can favor some

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environmental influences but not create them in any way (Rutter et al., 2001; Rutter, 2006). 3.3. The study of ‘‘nonshared environment” By the beginning of the 1980s, when behavior genetics had accumulated a sound empirical material, several authors had pointed out that the results of different twin studies revealed a similar pattern: environmental variance, especially when the study was relying on self-reported measures, tended to be rather withinfamily (nonshared), than between-family (shared). As described earlier, nonshared environmental variance represents all the nongenetic influences that result in differences between relatives while shared environmental variance represents all the nongenetic influences that result in relatives’ similarity. 3.3.1. Nonshared environment: the observation and the hypothesis Loehlin and Nichols (1976) were the first to emphasize the importance of the differentiating effect of the environment in their study of 850 twin pairs. Rowe and Plomin (1981) stressed the importance of nonshared environmental variance components in many behavioral genetic studies. However, the most influential work that has provoked a new area of empirical studies was the article by Plomin and Daniels (1987) in which the authors presented a literature review with a conclusion that greatest part of environmental variance is nonshared among almost all traits studied in behavior genetics. They also proposed a hypothesis that some specific objective factors must exist that make siblings in the same family different from one another (e.g., accidental factors, sibling interaction, family structure, differential parental treatment, and extrafamilial networks).3 It is necessary to emphasize that the hypothesis was not proved at that moment, although it has, unfortunately, been taken for granted by some authors (Rowe, 1994; Harris, 1995). The term ‘‘nonshared environment” has even migrated into some psychology textbooks, losing its hypothetical origin along the way (and failing to mention that it is only an anonymous component of phenotypic variance)—as if only the individual experiences of children are important for their development and not general familial factors such as socio-economic status or parenting styles (Pervin, 1996). Plomin suggested that testing the proposed hypothesis requires studying ‘‘more than one child per family”. Later, a three-step research program for studying the sources of nonshared environment was developed: (1) document differential experiences, which requires the construction of measures of the environment that are specific to each child in the family; (2) document the association between such differential experiences and differential outcomes; (3) investigate the extent to which associations between differential experiences and differential outcomes are causal (Plomin et al., 2001). This means that traditional methodology, which was employed during the pre-environmental period of behavior genetics, became insufficient. The contribution of objective environmental factors (see above for a definition) to effective environmental variances of a trait had to be investigated to determine what environmental factors really caused differences between siblings for the trait in question. Multivariate statistical methods became in demand to account 3 In fact, the hypothesis was formulated in an earlier paper (Rowe and Plomin, 1981), but it was not until 1987 that it really attracted the attention of psychologists.

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for the phenotypic and environmental variables (for a review on methodology, see Turkheimer and Waldron, 2000; Reiss et al., 2000; Rutter et al., 2001; Moffitt, 2005; Moffitt et al., 2005). In addition, studies of objective environmental influences made it desirable to use data not only from the target subjects (usually twins or siblings), but also from various other kinds of respondents, such as parents, peers, teachers, or psychologists (O’Connor et al., 1995). Since the end of the 1980s, a fair quantity of studies have been conducted, addressing the different steps of Plomin’s program for the research of nonshared environment and assessing the impact of different environmental factors on children’s individual outcomes. McGuire has discerned two general approaches that developed under the label of ‘‘nonshared environment”: (a) studies of individual experiences of children as such (experience-oriented approach) and (b) studies of the effects of objective environmental factors on children’s outcomes (outcome-oriented approach) (McGuire, 2001). From the behavior genetics’ standpoint, it is improper to speak about ‘‘environmental” influences if the genetic influences are not controlled. Therefore, the first approach should be named ‘‘nonshared experience” research; we will develop further only the second one—the behavior genetics’ approach. 3.3.2. Results of nonshared environment studies The most important works that summed up the results of more than a decade of nonshared environment research were published in the beginning of the 2000s in a review by Turkheimer and Waldron (2000), a book on the ‘‘Nonshared Environment and Adolescent Development” project by Reiss et al. (2000), and the ‘‘anniversary” article on the nonshared environment research (Plomin et al., 2001). Several studies were subsequently published (e.g., Asbury et al., 2003; Liang and Eley, 2005), and the area continues to develop, bringing ambiguous results. Turkheimer and Waldron (2000) presented a meta-analysis of 43 works on nonshared environment, concluding that, ‘‘Quantitative analysis of studies of specific nonshared environmental events shows that effect sizes measuring the effects of such variables on child outcomes are generally very small” (Turkheimer and Waldron, 2000). That is, despite the fact that the nonshared environmental variance component was significant (50% on average), the objective environmental influences explained less than 2% of the nonshared environmental variance in behavioral outcomes, regardless of the sample, methods, and studied traits. As Turkheimer and Waldron stated: ‘‘We emphasize that these findings should not lead the reader to conclude that the nonshared environment is not as important as had been thought. Rather, we believe that the appropriate conclusion is that the causal mechanisms underlying nonshared environmental variability in outcome remain unknown. The first candidate to receive serious consideration—objectively nonshared environmental events—does not appear likely to provide such an explanation, but it is important to remember that there are numerous other possibilities” (Turkheimer and Waldron, 2000). To date, we found several new studies of the nonshared environment (Deater-Deckard et al., 2001; Asbury et al., 2003; Pike and Atzaba-Poria, 2003; Liang and Eley, 2005). The authors of these studies conclude that, despite Turkheimer and Waldron’s pessimistic conclusions, several systematic nonshared environmental factors could be identified in the area of parental treatment of children, especially in the early ages and for the negative aspects of parental behavior. The results from the NEAD project (Reiss et al., 2000; Plomin et al., 2001), which aimed to discover the nonshared environmental influences on different behavioral outcomes of adolescents, deserve special attention. The study was planned in response to the

question posed by Plomin and Daniels in their seminar article (1987): ‘‘Why are children in the same family so different from one another?” (Reiss et al., 2000). The longitudinal project was designed to implement all three steps of Plomin’s program for studying the nonshared environment: to assess the differences in children’s experiences; to find the links between the experience and behavioral outcomes; and to determine the causal direction of these links. The sample for the NEAD study included more than 700 families with adolescent twins and siblings (for details, see Reiss et al., 2000). The measures covered three main areas: adolescents’ adjustment (competence and psychopathology) and subsystems of relationships in family. In addition, extrafamilial relationships were studied (Manke et al., 1995; Pike and Plomin, 1997). The data on each child were obtained from different informants (children, parents, and psychologist), and the multiple measures were combined to create reliable composite scales (Reiss et al., 2000). Comparing the results of the NEAD study with Plomin’s threestep program demonstrates that the aims of the first and second steps generally succeeded. For example, associations between parental negativity and child’s antisocial behavior were found (Reiss et al., 2000). The third step—longitudinal analysis—was not as successful because the studied characteristics appeared to be more or less stable. A multivariate analysis of associations between environmental characteristics (parental behavior) and adolescents’ adjustment revealed that most associations are genetically mediated. This finding led the authors to hypothesize on the central role of genotype–environment interplay in the adolescents’ development and in the functioning of family, named metaphorically a ‘‘relationship code” (Reiss et al., 2000). The main object of the NEAD project was the investigation of objective sources of nonshared environmental variance in children’s adjustment. Reiss and colleagues state that, despite the fact that nonshared environmental variance appeared to be the most important non-genetic variance component for five of the seven composite measures of behavior, the search for objective environmental influences has brought almost nothing. They conclude: ‘‘. . .our search for specific, non-genetic, nonshared environmental influences has been frustrating. In adolescence this aspect of the environment does not appear to reside in differential parental treatment or asymmetrical sibling relationships (nor, according to our preliminary measures, in different qualities of peer-group relations)” (Reiss et al., 2000). This conclusion is generally in line with the results of meta-analysis described above (Turkheimer and Waldron, 2000). 3.3.3. The perspectives of nonshared environment research After two decades of empirical studies of nonshared environment in behavior genetics, the viewpoints of different scientists have become polarized. One group of authors takes an optimistic stance (Plomin et al., 2001). In particular, from Plomin’s point of view, the search for nonshared environment should be continued with the aid from new, more sophisticated measures and bigger samples (Plomin et al., 2001; Asbury et al., 2003). Other authors adhere to a more pessimistic position towards the possibilities of identifying the objective sources of nonshared environment by means of the methodology established in contemporary behavior genetics (for a review, see Turkheimer and Waldron, 2000). According to Turkheimer, a possibility exists that ‘‘. . .objectively nonshared environmental events are indeed the source of nonshared variability in outcome, but the causal impact of any single environmental event is very small and unsystematic; it is only the cumulative effect of a multitude of small environmental differences that cause detectable outcome differences among siblings” (Turkheimer and Waldron, 2000). Turkheimer predicts that the nonshared environment studies are doomed to the ‘‘gloomy

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prospect”, since the accepted mainstream statistical models do not permit to single out the environmental influences (Turkheimer and Waldron, 2000; Turkheimer, 2000, 2004, 2006). He sees a solution for the posed problem in the change of methodology of data analysis from linear to non-linear models (Turkheimer, 2004). 3.4. The studies of environment as a mediating context for development 3.4.1. The findings on shared environmental influences The assertion that the most important influences on the psychological development are nonshared (Plomin and Daniels, 1987) has been criticized by developmental psychologists (e.g., Hoffman, 1991) and gradually called into question in behavior genetics. Numerous studies have proven that the observation made by Plomin and his colleagues is not that ubiquitous. Furthermore, it is mainly limited to results from studies relying on self-reports. For some behavioral traits, the shared environment is the important variance component; these include antisocial and delinquent behavior (Miles and Carey, 1997; Rhee and Waldman, 2002), attitudes (Abrahamson et al., 2002), positive emotionality (Tellegen et al., 1988; Bergeman et al., 1993; Riemann et al., 1997), and additional behavioral phenotypes. As shown in studies conducted by Walden et al. (2004) and Rose et al. (2003), the shared environmental factors are important for the development of psychopathology and competences in adolescents. The results from the NEAD study also show high estimates of shared environment for the various characteristics of family interactions and adolescents’ adjustment (Bussell et al., 1999; Neiderhiser et al., 1999; Reiss et al., 2000; Spotts et al., 2001). Indeed, when the results of the NEAD study were reanalyzed, taking into account the measurement errors, they contradicted the initial assumption; the nonshared environmental components almost vanished from the variance, leaving room for genetic and shared environmental contributions (Loehlin et al., 2003). 3.4.2. Environmental differences and heritability Recently, a special area of behavioral genetic studies of measured environmental influences have emerged. The evidence was obtained for the effect of such variables as family socio-economic status (SES), previously neglected in behavioral genetic studies as ‘‘shared”, and thus unimportant, on the relative contribution of genetic and environmental variance components. For example, it was shown that the level of education in parents affects the heritability of IQ in children (Rowe et al., 1999). Similar results have been reported in a study of influence of SES on heritability of IQ in children (Turkheimer et al., 2003). The methodology and results of this special kind of studies—behavior genetic studies of environmental influences on heritability—is well described in the works conducted by Rutter and collaborators (Moffitt et al., 2006; Rutter, 2007; Rutter et al., 2006). 3.4.3. Measured shared environment The impact of social context and family relationships on psychological functioning of adolescent twins was demonstrated in the works conducted by Crosnoe and Elder (2002) as well as by Horwitz et al. (2003). The differentiating effect of objectively shared life events on MZ twins’ personality traits was revealed in the Torgersen and Janson study (2002). Turkheimer et al. (2005) have pointed out that studies of shared environment have their own methodological problems. They emphasized that statistical algorithms, which are widely accepted in behavior genetics, can lead to incorrect conclusions when applied to data on shared environmental factors. Therefore, such studies should be interpreted with caution (for details, see Turkheimer et al., 2005).

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3.4.4. The role of observational methods and experiments: Implications for the research of the environment One important observation made by several authors is that behavioral genetic studies using self-reports and observational methods yield different patterns of results. In particular, the contribution of shared environmental variance increases when observational methods are used (Plomin’s thesis was built upon the results of self-reports). The review of behavior genetic studies of aggression by Miles and Carey (1997) and the special review of observational studies in behavior genetics by Borkenau et al. (2000) conclude that such studies yield unusually high estimates of shared environmental variance. The results of the ‘‘German Observational Study of Adult Twins” (GOSAT) also prove the importance of the shared environment (Borkenau et al., 2001). Observational data from the NEAD project are consistent with these studies as well, providing estimates of shared environment as high as 30%; such estimates appeared to be stable during the three-year period (Spotts et al., 2001; Loehlin et al., 2003). Two similar studies of attachment in twins that were conducted in the laboratory context using observational methods have equally similar and surprising results: one half of variance was attributed to nonshared environmental factors, and another half to the shared environment, with no significant genetic effect (Bockhorst et al., 2003; O’Connor and Croft, 2001). The experimental study of the heritability of cognitive characteristics in 6- to 7-year-old twins was conducted by Malykh et al. (1998) who demonstrated that the relative contribution of genetic and environmental components to variance in cognitive characteristics changes if the structure of the subject’s activity during the solving of the cognitive task is transformed by the training. Prior to the training, the significant variance components were genetic (53%) and nonshared environmental factors (47%); after the training, they were nonshared environment (48%) and shared environment (52%) (Malykh, 2004a). Thus, the observational and laboratory studies present different kinds of results: the shared environmental component is more important. Borkenau and colleagues suggested an explanation for this phenomenon. Perhaps genetic effects are more easily revealed on the higher levels of generalization of behavior. The more concrete and situational the psychological or behavioral variable is, the lower the genetic contribution is and the higher the role of shared environment is. Although self-reports usually assess generalized personality traits, laboratory studies allow for an examination of the more situational forms of behavior that are more prone to environmental influences (Borkenau et al., 2000). To conclude, the influences of the environmental context may appear to be more detectable in laboratory and rather elusive in the self-report studies of general and decontextualized personality traits.

4. Outside behavior genetics: psychological and psychoanalytic studies of twins, their environment and experience Meanwhile, in parallel with the early development of behavior genetics, the twins—the main participants of behavior genetic research—and their family environment, were thoroughly studied in a quite different field of psychology, psychoanalysis. The first works on psychology of twins and siblings were conducted mostly within the framework of case studies from the 1930s until the 1960s (for a review, see Stewart, 2000). As described by Hartmann in 1954 (cited in Stewart’s review), the twins are always significantly affected by the personality of each other, and this special social process was termed ‘‘mutual identification”. Burlingham in the 1940s observed four sets of twins and one set of triplets during several years and described the development of their psychological reality and personality (Burlingham, 1952). Leonard (1961) studied

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the process of individuation in twins, also from the psychoanalytic perspective, concluding that intertwin relationship differs from most other personal interrelationships because of certain characteristics peculiar to intertwin identification. The studies of ‘‘twinning reaction” (Joseph, 1959, 1961; Joseph and Tabor, 1961) are also an example of the detailed analysis of psychoanalytic treatment of twin pair. Close ideas were expressed by Ortmeyer in his concept of ‘‘we-self” in twins (Ortmeyer, 1970). Also, Winestine (1969) studied sibling rivalry and other social processes within the twin pair, and Glenn (1966) studied relationships in opposite-sex twins. By the end of the 1970s, the relationships in siblings’ pairs had become an object of systematic investigation in the academic area of psychology (Zazzo, 1960, 1976, 1978; Lytton, 1980). Thus, an interesting study of adolescent and adult twins’ identity and relationships was conducted in the early 1980s by Ainslie (1997), based on the psychoanalytic framework and qualitative interview methodology. The in-depth qualitative studies of personal experience (psychoanalysis is one example of this kind of work) became less fashionable after the introduction of e-mail based questionnaires and impressive statistical modeling. Nowadays, it appears that differential psychological methodology is severely limited in its capabilities of explaining the very phenomena discovered within its own framework. As Ainslie wrote: ‘‘Newman et al. (1937), for example, in discussing their failure to find significant results with a series of personality tests, conclude that the difficulty may be due to ‘‘subtle” characteristics of human relationships that ‘‘are revealed only by individual analysis”. A clinical or descriptive method makes such individual analysis possible in a way that more traditional methods of research do not” (Ainslie, 1997).

5. Beyond the genes and environment: behavior genetics and the dynamics of human experience Behavior genetics is being currently renewed in both theory and methodology. The movement towards renewal is evident from two sides: from ‘‘inside” behavior genetics (Rutter, 2006) and ‘‘outside”, especially, from developmental psychology (Bronfenbrenner and Ceci, 1994; Coll et al., 2004). We could say that studies on environmental factors in behavior genetics are undergoing a soft paradigm shift into a new period of development. This is the result of a long period of empirical studies designed according to the rules of standard biometrical approach that have forced scientists to search for new theories or methodology and for alternative understanding of the genotype–environment interplay. The opposition between genes and environment discussed at scientific debates has been softened (Reiss et al., 2000; Rutter, 2006, 2007). The significant hypothesis on nonshared character of environmental influences (Plomin and Daniels, 1987) still appears unproven (Reiss et al., 2000). At the same time, it has attracted the attention of developmental psychologists to behavior genetic studies. This, in turn, has led to an understanding that genetic factors should be explicitly incorporated into theoretical and methodological schemes of developmental psychology (Maccoby, 2000; Collins et al., 2000). The results of environmental studies in behavior genetics show that for many psychological traits the genotype–environment interactions are the rule rather than the exception (Rutter et al., 2001). This leads behavior genetics to a rethinking of the crucial assumptions that underlie the methodology of classic biometrical studies. Turkheimer proposed that the use of non-linear methods may become a remedy to the ‘‘gloomy prospect” of the studies of environment (Turkheimer, 2004; Turkheimer et al., 2005).

From the simplified two-factor scheme with all the concurrent conventionalities and limitations, behavior genetics is turning to multilevel models of interactions of genetic, personal, and environmental factors that reveal themselves differently depending on the context of the study. The notion of genes and environment as the two main causes of individual differences gives rise to the systemic view of causation of the phenomena under study (for details, see Rutter et al., 2001; Rutter, 2006). The heritability coefficient h2, which was previously used as the quantitative measure of genetic effects, is now being considered as an intermediate unit in studies that include the measurements of concrete environmental influences (Bronfenbrenner and Ceci, 1994; Moffitt, 2005; Moffitt et al., 2005). Behavior geneticists are searching for adequate methods of gathering and processing the data. The inclusion of environmental factors in phenotype has led the study outside the individual personality into the field of social interactions and relationships. The difficulties in identifying the mechanisms and factors of environmental influences are stimulating scientists to employ alternative theories and methods previously ignored within behavior genetics—namely, psychoanalysis and qualitative research methods (see Reiss et al., 2000). As Reiss et al., propose, the problem of ‘‘nonshared environment” might reveal itself really as the problem of the ‘‘nonshared representations” of experience. This means that first-person accounts of how people experience things and events in their lives, for example, narratives (Polkinghorne, 1988), are of primary importance for psychologists interested in the sources of individual differences. It is not some special objectively nonshared environment influences the development and makes identical twins different, that has to be discovered, but the very nature of human consciousness is working that way to build the individual representations of the environment (inseparably from self-representations), which, of course, will be nonshared. In a similar fashion, McGuire proposed that the study of an unexplored phenotype in behavior genetics such as self and identity might be of value for the explanation of nonshared environmental factors on the development (McGuire, 2001). The reconceptualization of environment as different levels of contexts is driving behavior genetics closer to formerly juxtaposed systemic approaches to development (Lerner, 2004). ‘‘The former paradigm, that was limiting itself with the genetic explanations, did not result in significant progress in understanding the nature of behavior” (Malykh, 2004b). The shift from the static understanding of psychological variables as fixed traits to more complex structural concepts of behavior, which are based on the notion of goaldirected, socially mediated and intentional activity, leads behavior genetics to a new interactional view of genotype–environment interplay contrary to the outdated additive model (Rutter, 2006). One conclusion from the environmental studies of behavior genetics is that sibling and twin designs have proven to be the important instrument for revealing unique features of the environmental influence. In particular, one valuable strategy is to investigate the effects of different types of environment on a trait while controlling for genetic factors (Turkheimer et al., 2003). Another strategy is to actively control and construct the environmental influence in the experimental situations (Malykh, 2004a), but that is possible because of practical and ethical reasons. In this perspective, the behavior genetics methods become an important tool for studying environment. Nevertheless, the results from studies of nonshared environment are puzzling. Obviously, there is a problem with finding the actual sources of individual differences by means in individual differences-oriented approach in behavior genetics. Perhaps, now it is time for the science of genes and environment to rediscover the value of first-person narrative accounts of human experience (Bruner, 1990; Polkinghorne, 1988) with more scientific rigor and clarity,

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