The association between the COMT gene Val158Met polymorphism and preschoolers’ autobiographical memory details and narrative cohesiveness

Cognitive Development 39 (2016) 181–188

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Cognitive Development

The association between the COMT gene Val158 Met polymorphism and preschoolers’ autobiographical memory details and narrative cohesiveness P. Tõugu a,∗ , M. Vaht a , T. Tulviste a , T. Veidebaum b , J. Harro a a b

University of Tartu, Institute of Psychology, Näituse 2, 50409 Tartu, Estonia National Institute for Health Development, Hiiu 42, 11619 Tallinn, Estonia

a r t i c l e

i n f o

Article history: Received 25 July 2015 Received in revised form 28 May 2016 Accepted 6 June 2016 Keywords: Longitudinal study COMT gene Episodic memory Narrative cohesiveness Preschoolers

a b s t r a c t A longitudinal study addressed the effect of COMT gene Val158 Met polymorphism on memory narratives of children. The effects of sex and maternal education were also studied. The total sample consisted of 486 children (48.9% boys), who were 4 years old during the first and 6 six years old during the second wave. COMT gene Val158 Met polymorphism had an effect on the number of memory details in autobiographical memories with Met/Met homozygotes providing more details than the children with the Val/Met genotype. A genotype by maternal education interaction was observed at the age of four, but not at the age of six: Met/Met genotype children in the lower maternal educational attainment group had more detailed memories at the age of four than their Val/Met counterparts. © 2016 Elsevier Inc. All rights reserved.

1. Introduction The COMT gene Val158 Met polymorphism has been implicated in several mental disorders, and the effects of the COMT gene Val158 Met polymorphism on the cognitive functions of normal adult population are well established (Tunbridge, Harrison, & Weinberger, 2006). At the same time, fewer studies take a developmental approach and incorporate the environmental factors that could moderate the genetic effects. The present study is a longitudinal study that aims at establishing the effect of the COMT gene Val158 Met polymorphism on such an everyday cognitive task as providing autobiographical memory narratives in a sample of normally developing preschool children. The variation in the children’s environment stemming from differences in maternal education is also considered. 1.1. COMT and cognitive functions Cathechol-O-methyl transferase (COMT) is an enzyme that is involved in the catabolism of catecholamines, including dopamine and has a particularly important role in the functioning of prefrontal cortex (Tunbridge, Bannerman, Sharp, & Harrison, 2004). The COMT gene contains a functional polymorphism in codon 158, where methionine (Met) can be substituted for valine (Val). The Met form of the COMT enzyme is less active and leads to higher levels of synaptic dopamine in the prefrontal cortex (Chen et al., 2004; Lotta et al., 1995). As the COMT gene is important for modulating dopamine levels

∗ Corresponding author. E-mail address: [email protected] (P. Tõugu). http://dx.doi.org/10.1016/j.cogdev.2016.06.002 0885-2014/© 2016 Elsevier Inc. All rights reserved.

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in the prefrontal cortex, it has been related to several cognitive tasks relying on this brain region (e.g., Caldú et al., 2007; Tunbridge et al., 2006; for review). For example, in representative adult samples, the COMT gene Val158 Met polymorphism has been related to individual differences in executive functions (e.g., Bruder et al., 2005; Starr, Fox, Harris, Deary, & Whalley, 2007) and also to episodic memory (De Frias et al., 2004), and IQ (Barnett, Scoriels, & Munafò, 2008). In general, the Met/Met genotype is associated with better results and the Val/Val genotype with lower performance on cognitive tasks (Bruder et al., 2005; Starr et al., 2007). At the same time, it must be noted that a meta-analysis of the effects of the COMT genotype on cognitive functions suggests that despite numerous studies COMT Val158 Met polymorphism seems to have little effect on cognitive functions (Barnett et al., 2008). Among adolescents such a complex task as reading comprehension has been shown to be influenced by the COMT gene (Grigorenko et al., 2007). In child samples, the COMT gene has been related to attention in toddlers (Voelker, Sheese, Rothbart, & Posner, 2009), and to personality traits (DeYoung et al., 2011), executive functions (Barnett, Heron, Goldman, Jones, & Ke, 2009; Diamond, Briand, Fossella, & Gehlbach, 2004; Dumontheil et al., 2011), and verbal IQ (Barnett et al., 2007) in older children. Toddlers carrying a Val allele showed better attention than Met/Met homozygotes, but only when they received high quality parenting suggesting a role of supportive environmental conditions; the attention of Met/Met homozygotes was not influenced by parenting quality (Voelker et al., 2009). Met/Met homozygotes received higher scores on the Openness/Intellect personality trait in the sample of 608 8–13-year-old disadvantaged children (DeYoung et al., 2011). At the age of 8 and 10, the Met allele carriers in a population cohort study received higher scores in working memory and verbal IQ tests (Barnett et al., 2007). COMT activity has been shown to increase with age reaching its peak in adulthood (Tunbridge et al., 2007). Therefore, children’s age is an important factor in developmental studies. Indeed, there is evidence that the advantages of Met/Met homozygotes in working memory tasks only become apparent at the age of 10–12 (Dumontheil et al., 2011) and that the effect of genotype on verbal IQ is stronger after children reach puberty (Barnett et al., 2007). The COMT gene affects cognitive functions of men and women differently (Harrison & Tunbridge, 2008; White et al., 2014). In large population-based child samples, the COMT genotype has been shown to have very little effect on girls’ performance while in the boys’ group the effect on executive functions and verbal IQ is notable (Barnett et al., 2009, 2007). Barnett et al. (2007) found that in the sample of boys a difference of 2–10 points of verbal IQ was attributable to the genotype at the age of 10 with Met/Met homozygotes receiving the highest scores, Val/Val homozygotes the lowest, and Val/Met heterozygotes placed in between. In a study investigating other loci besides Val158 Met, Barnett et al. (2009) found a linear relationship between COMT haplotype and verbal IQ for boys but no such relationship for girls in a sample of 8-year-olds. The effects of COMT gene Val158 Met polymorphism on cognitive tasks are mediated by external factors, such as educational attainment (Enoch, Waheed, Harris, Albaugh, & Goldman, 2009). Enoch et al. (2009) found that on several WAIS-R subscales, the score of Met allele carriers among American Indians improved alongside the number of years of education while there was practically no change in the Val/Val genotype group. Comparison of the subsamples based on educational attainment showed that the Met allele carriers performed worse than the Val/Val homozygotes in the low educational attainment group, whereas higher-educated Met allele carriers performed better. The crossover of effects was at 11–12 years of education. In addition, maternal rejection has been shown to interact with COMT gene genotypes to predict reading comprehension difficulties in incarcerated adolescents (Grigorenko et al., 2007). Similarly, parenting quality interacts with COMT gene genotypes to affect attention in toddlers (Voelker et al., 2009). These results imply that the COMT gene Val158 Met polymorphism exhibits different vulnerability to environmental factors, which could be of great importance in childhood. The COMT gene has been proposed as a candidate gene affecting episodic memory (Koppel & Goldberg, 2009) because it influences the dopamine system involved in the cognitive functions of prefrontal cortex. There is evidence that dopamine and COMT gene Val158 Met polymorphism are involved in the formation of episodic memories (Schott et al., 2006). The Met/Met homozygotes perform better in episodic memory tasks than the Val allele carriers, and the difference is especially prominent in recall tasks (De Frias et al., 2004). The effect of the COMT gene on autobiographical memories has not been addressed. 1.2. Autobiographical memory and narrative Autobiographical memory is episodic memory of past experience that is autonoetic and forms a part of declarative memory (Tulving, 2002; Wheeler, Stuss, & Tulving, 1997). Autobiographical memory develops during the preschool years and is influenced by other cognitive attainments of the period including improving language skills and narrative development (Fivush & Nelson, 2006; Nelson & Fivush, 2004). Children younger than four years are generally not considered to have a fullyfunctioning episodic or autobiographical memory system (Nelson, 1993; Wheeler et al., 1997). Episodic memory encoding and retrieval involve the frontal lobes, especially the prefrontal cortex (see Tulving, 2002; for review), and autobiographical episodic memory engages several areas of this brain region (Levine et al., 2004). Narrative ability is a sophisticated language ability that starts to develop as the elementary language abilities are acquired and continues to improve until or throughout adulthood. This complex skill involves temporal organization of the structural elements of the story requiring careful planning and close monitoring of one’s narrative production. Narrative comprehension and production have been considered to reflect executive functions in children (Pureza, Gonc¸alves, Branco, Grassi-Oliveira, & Fonseca, 2013; Purvis & Tannock, 1997) and shown to be related to various other executive functions tasks (Dodwell & Bavin, 2008; Duinmeijer, de Jong, & Scheper, 2012; Friend, Bates, Nobuyuki, & Coldren, 2014). Narrative production involves

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the same network of frontal lobe areas implicated in executive functions (Mar, 2004); therefore, one could expect it to be affected by the dopamine system and the COMT gene Val158 Met polymorphism. The development of autobiographical memory in preschool years co-occurs with the development of language, selfconcept, concept of time, narrative etc. and is affected by the socialization of children. Thus, the way parents reminisce with children about past events shapes the way children come to talk about memories with others (Nelson & Fivush, 2004). The major inference in this line of research is that children with more elaborative mothers report more detailed autobiographical memories (e.g. Reese, Haden, & Fivush, 1993). Higher socioeconomic status, often operationalized as higher maternal education, is linked to children’s better language and narrative abilities. University-educated mothers talk more to their children, provide more opportunities for decontextualized language use (Hoff, 2006), and are more likely to have an elaborative reminiscing style (Fivush, Haden, & Reese, 2006). These characteristics support language development and early autobiographical memory (Jack, MacDonald, Reese, & Hayne, 2009). Gender is also related to autobiographical memory and narrative abilities. Several studies indicate that women recall more details of autobiographical memories than men (MacDonald, Uesiliana, & Hayne, 2000; Wang, 2013; Wang, Hou, Tang, & Wiprovnick, 2011). Similarly, girls tend to produce more complex and cohesive narratives than boys during preschool years (Fivush, Haden, & Adam, 1995). 1.3. Present study The present study is a longitudinal observational study to establish the effect of the COMT genotype, maternal education, and gender on the memory narratives of preschool children. The possible effect of COMT gene Val158 Met polymorphism on such cognitive functions as autobiographical memory and narrative skills have not been addressed. Although COMT effects are often modified by other factors (Tunbridge et al., 2006), few studies have included environmental variables such as educational attainment or parenting quality in their design. In the present study, COMT gene Val158 Met polymorphism is expected to influence children’s narrative accounts of their autobiographical memories. Based on the previous research on adults, Met/Met homozygotes are expected to exhibit better autobiographical memory (in providing more details) and to provide more cohesive narratives than the Val allele carriers. Girls are expected to recall more details and to provide more cohesive narratives. A gender by genotype interaction is expected: the genotype effect is predicted to be more prominent in the sample of boys as compared to girls. Maternal education is expected to interact with the genotype effect. 2. Method 2.1. Participants All children were participants in the large-scale longitudinal Estonian Children Personality, Behavior, and Health Study (Tomson, Merenäkk, Loit, Mäestu, & Harro, 2011). The present sample uses a new cohort of participants and includes children who participated in the additional block devoted to the investigation of memory narratives. Participants were recruited from kindergartens all over Tartu (the second largest town in Estonia with a population of 100 000) in 2007/2008 and again in 2009/2010. All participants were Caucasian and full-time attendees of an Estonian-speaking kindergarten. Four hundred and fifty two children were recruited for the subsample in the first wave of the study. Narrative and autobiographical memory information is available for 409 of them (48.2% boys; mean age 4.24, range 3–5.4). More children were recruited during the second wave yielding a total sample of 486 children (48.9% boys). Narrative and autobiographical memory data of the second wave are available for 360 children (180 boys, 180 girls; mean age 6.11, range 3.8–7.3); 270 of them also participated in the first wave. Educational attainment was known for 463 mothers: 39 mothers had less than a high-school education, 201 had a high-school diploma, and 223 mothers had a university degree. Due to the low number of mothers without a high-school education two groups were used for analyses: university-educated mothers (including all mothers with at least a bachelor’s degree) and high-school or less educated mothers. Blood samples for genotyping were obtained from 384 participants. The group of children whose blood samples were not obtained did not differ from the genotyped children in terms of age, their maternal education, or gender. The number of participants grouped by gender, maternal educational attainment, and genotype are presented in Table 1. 2.2. Genotyping Genomic DNA was extracted from dried blood spots using Qiagen QIAamp® DNA Mini Kit(Qiagen, Hilden, Germany). Genotyping reactions of COMT Val158 Met polymorphism (rs4680) were performed in a total volume of 20 ␮l with 10–50 ng of template DNA. The real-time polymerase chain reaction (RT-PCR) was performed with primers and fluorescent probes obtained from Applied Biosystems (Foster City, CA, USA) Custom TaqMan SNP Genotyping Assays. RT-PCR reaction components and final concentrations were as follows: 1:5 5 × HOT FIREPol Probe qPCR Mix Plus (ROX) (SolisBiodyne) and 1:20 80 × TaqMan Primers Probe (F 5 -CCCAGCGGATGGTGGAT-3 ; R 5 -CAGGCATGCACACCTTGTC-3 ; Reporter 1-TTCGCTGGCATGAAG (VIC); Reporter 2-TCGCTGGCGTGAAG (FAM)). Reactions were performed on the ABI 7500 Real-Time PCR system and the amplification procedure consisted of an initial denaturation step at 95 ◦ C for 15 min and 40 cycles of

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Table 1 Number of participants grouped by gender, maternal education, and genotype at both waves.

Wave 1

Gender

Maternal education

Met/Met (n)

Val/Met (n)

Val/Val (n)

Girls

Less than a university degree University degree or higher Less than a university degree University degree or higher

30 19 20 23

47 43 45 35

14 19 13 21

Less than a university degree University degree or higher Less than a university degree University degree or higher

25 19 17 20

32 37 41 33

14 13 13 21

Boys Wave 2

Girls Boys

Genotype

95 ◦ C for 15 s and 60 ◦ C for 1 min. All genotyping reactions were carried out in duplicates and extra negative controls were added to each reaction plate. No inconsistencies occurred. Genotypes were found to be in the Hardy-Weinberg equilibrium. 2.3. Autobiographical memory and narrative ability 2.3.1. Procedure Children were interviewed in the kindergarten individually in a separate room by a female research assistant. First, the Comprehension Scale from the Reynell language test (Edwards, Garman, Hughes, Letts, & Sinka, 1999) was administered. The Comprehension Scale of the test ends with questions about a picture depicting people in a burger bar celebrating a birthday. The picture served as a warm-up task and children were encouraged to engage in talk about the picture with the research assistant after the test. The children were then asked to provide accounts of two past events (their own previous birthday and the past weekend). Two events were selected in order to obtain a more adequate picture of children’s ability to remember past events and to counterbalance possible recency effects regarding birthday recounts. The children were invited to talk about their previous birthday with the standard prompt “Do you remember your last birthday party? Can you tell me something about it?” The research assistant was instructed to provide positive feedback and backchannel responses such as “yeah”, “really?”, “and then?” as in previous studies (Peterson, Jesso, & McCabe, 1999). When the child did not provide any more information about the birthday party, the children were asked to talk about the past weekend with a standard question “What did you do with mom and dad at home this weekend?” A more detailed description of the procedure is available in Tõugu, Tulviste, & Suits (2014). The interaction was audio-taped and transcribed. The memory narratives were coded for the amount of content information provided (autobiographical memory details) and narrative cohesiveness. 2.3.2. Autobiographical memory details The number of details provided was used as the indicator of autobiographical memory. A detail code was assigned to each subject-verb combination carrying content information that was provided by the child. In addition, one-word meaningful answers given by the child were also ascribed a detail code. The cumulative count of details was used in the analyses. Reliability of coding was assessed on twenty percent of the transcripts; the intraclass correlation between two raters was 0.99. 2.3.3. Narrative cohesiveness Cohesiveness was used as an indicator of narrative ability. Cohesiveness was rated on a scale ranging from 0 (the account includes no links between pieces of information and no orienting information about temporal sequence or spatial orientation) to 4 (meaningful links between the pieces of information including such conjunctions as but, therefore, etc. and orienting information about the event provided). The two memory narratives (birthday and weekend) were rated separately, and the better of the two scores was used in the analyses as an indicator of child’s ability to provide cohesive memory accounts at a given time. Reliability of coding cohesiveness was assessed on twenty percent of the transcripts; Cohen’s Kappa was 0.84. 3. Results 3.1. Descriptive statistics and outline of analyses Genotype information was available for 384 children (187 boys, 197 girls). There were 106 (28%) Met/Met homozygotes (48 boys, 58 girls), 198 (51%) Val/Met heterozygotes (97 boys, 101 girls), and 80 (21%) Val/Val homozygotes (42 boys, 38 girls). The means and standard deviations of the number of episodic memory details and the narrative cohesiveness scores for all of the groups are provided in Table 2. SPSS Mixed linear models followed by univariate ANOVA analyses were used to study the longitudinal effects of COMT, gender, and maternal education on children’s autobiographical memory and narrative cohesiveness separately. SPSS Statistics version 22 was used for the analyses.

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Table 2 Means for the number of episodic memory details and narrative cohesiveness by gender and genotype. Genotype I Wave Met/Met Val/Met Val/Val II Wave Met/Met Val/Met Val/Val

Autobiographical memory details: M (SD)

Narrative cohesiveness: M (SD)

Boys (N = 43) Girls (N = 49) Boys (N = 83) Girls (N = 93) Boys (N = 34) Girls (N = 33)

17.00 (9.51) 14.02(10.57) 14.66 (8.85) 14.02 (9.38) 15.53 (8.59) 12.36 (6.32)

1.98 (.83) 1.85 (.80) 1.82 (.68) 1.83 (.75) 1.71 (.63) 1.85 (.62)

Boys (N = 37) Girls (N = 44) Boys (N = 77) Girls (N = 69) Boys (N = 34) Girls (N = 27)

22.00 (14.34) 20.77 (12.45) 21.03 (11.62) 20.33 (12.44) 18.97 (10.07) 19.15 (13.37)

2.35 (.98) 2.45 (.95) 2.30 (.92) 2.71 (.91) 2.41 (.92) 2.35 (.94)

Prior to the analyses correlations between autobiographical memory details and narrative cohesiveness and the possible co-variates such as the recency of the child’s birthday, experimenter’s prompts, and the Reynell language comprehension score were studied. Controlling for the current age of the children, research assistant’s prompts were correlated with autobiographical memory details (r = 0.35 for the first wave and r = 0.19 for the second wave, p < 0.05) and the Reynell language comprehension score with narrative cohesiveness (r = 0.13 for the first wave and r = 0.13 for the second wave; p < 0.05) during both waves. These variables are included as covariates in the respective analyses.

3.2. Autobiographical memory details A Linear Mixed Models analyses for repeated measures was carried out with children’s autobiographical memory details as the dependent variable, gender, the COMT genotype, and maternal education as factors, and children’s age and research assistant’s prompts as covariates. Gender by the COMT genotype, gender by maternal education, and the COMT genotype by maternal education interactions were also included in the analyses. The model indicated a significant effect of the COMT genotype on children’s autobiographical memory details (F (2; 328) = 3.09, p < 0.05) and a significant COMT genotype by maternal education interaction (F (2; 328) = 4.92, p < 0.01). Both covariates, children’s current age (F (1; 413) = 81.99, p < 0.001) and research assistant’s prompts (F (1; 553) = 61.1, p < 0.001), had a significant effect on autobiographical memory details. Pairwise comparison with the Bonferroni correction indicated that children with the Val/Val genotype provide fewer autobiographical memory details than the Met/Met homozygotes (p < 0.05). The COMT genotype by maternal education interaction was studied separately for both waves (the means and standard errors for the groups are plotted in Fig. 1). For the first wave, the post hoc test with the Bonferroni correction indicated that the mean number of autobiographical memory details is significantly different for the Met/Met homozygotes and the Val/Met heterozygotes among the children whose mothers have an education less than a university degree. No differences between the groups appeared during the second wave.

Fig. 1. Genotype by maternal education interaction on the autobiographical memory details during both waves.

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Fig. 2. Genotype by maternal education interaction on the narrative cohesiveness score during the two waves.

3.3. Narrative cohesiveness A Linear Mixed Models analysis for repeated measures was carried out with children’s narrative cohesiveness as the dependent variable, gender, the COMT genotype, and maternal education as factors and children’s age and the Reynell Comprehension Score as covariates. Gender by the COMT genotype, gender by maternal education, and the COMT genotype by maternal education interactions were also included in the analysis. The model indicated a significant COMT genotype by maternal education interaction (F (2; 329) = 3.92, p < 0.05). Both covariates, children’s current age (F (1; 543) = 18.51, p < 0.001) and the Reynell Comprehension Scale score (F (1; 435) = 6.42, p < 0.05), had a significant effect on narrative cohesiveness. The COMT genotype by maternal education interaction was studied separately for both waves (the means and standard errors for the groups are plotted in Fig. 2). The post hoc test with the Bonferroni correction indicated no significant differences between the groups for the two waves separately. 4. Discussion The present study set out to establish the effect of COMT gene Val158 Met polymorphism on such an everyday cognitive task as providing autobiographical memory narratives in a sample of normally developing children. The effects of gender and the environment (operationalized as maternal education level) were also studied. The present study indicates that the genotype is related to individual differences in autobiographical memory during the preschool years as the Val/Val homozygotes provided significantly fewer details of their past experiences than the Met/Met homozygotes. The direction of the effect is well in line with other studies involving children were Met/Met homozygotes outperform their counterparts in cognitive tasks (e.g. Diamond et al., 2004) or receive higher scores on the Openness/Intellect trait or verbal IQ measure (Barnett et al., 2007; DeYoung et al., 2011). Most prior studies have included older children; therefore, the study extended the age range and the scope of cognitive functions that the COMT gene Val158 Met polymorphism has been shown to influence. Contrary to the expectations the COMT gene Val158 Met polymorphism did not have an effect on children’s narrative cohesiveness. It is possible that the five-point Likert-type scale used here to score narrative cohesiveness does not fully cover the nuances of narrative development. The growing number of mental tasks affected by the COMT gene polymorphism raises the questions of the COMT gene having a general effect on cognitive functions instead of specialized effects on particular skills and abilities. Also, autobiographical memory development has been shown to be related or determined by general cognitive development in childhood (Picard, Reffuveille, Eustache, & Piolino, 2009). The autobiographical memory and narrative production measurement used in the present study could implicate several executive functions and working memory skills. Therefore, the observed genotype effect could be somewhat cumulative or restricted to higher-order functions that are involved in the measured tasks. Previous research has implied that especially higher-order tasks are the ones that are modulated by the COMT gene Val158 Met polymorphism (Bruder et al., 2005). Yet, this question could not be answered by the present study, and it remains for the future studies to disentangle the particular cognitive functions and skills affected by the COMT gene Val158 Met polymorphism in this age group. The autobiographical memory and narrative ability were expected to be affected by children’s gender. Surprisingly, this was not the case as there were no significant differences in the number of autobiographical memory details that boys and girls provided and in the cohesiveness rating of their memory narratives. Previous studies have shown that the performance of boys is more influenced by the genotype than that of girls (Barnett et al., 2009, 2007), and that men who have the Met/Met genotype seem to be more susceptible to certain mental disorders (Perkovic et al., 2013; Tunbridge et al., 2006). The present study did not reveal any gender by genotype interactions. At the same time, it must be noted that the studies showing boys

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to display stronger genotype effects involved children aged 8–12 years. Therefore, there is a possibility that the gene by gender interaction could be related to a later age period. Based on previous studies we expected the environmental differences stemming from maternal educational attainment to affect children’s developing cognitive skills. There was no main effect of maternal education which could be due to the limited range of education in the sample (as there were very few children of mothers without a high-school education, they could not be studied separately). Yet, there appeared a maternal education by genotype interaction effect on autobiographical memory and narrative cohesiveness. The amount of details provided in the autobiographical memories of Met/Met homozygotes and Val/Met heterozygotes were differently influenced by maternal educational attainment: the Val/Met heterozygotes of mothers with a lower educational attainment provided fewer autobiographical memory details than their Met/Met counterparts during the first wave. During the second wave the difference had disappeared. Prior research has indicated that in the American Indian adult sample the Met/Met genotype group is susceptible to the influence of environmental factors such as education (Enoch et al., 2009). At the same time, Voelker et al. (2009) found that in the sample of toddlers, the outcomes of Val allele carriers were influenced by parenting quality. The results of the present study are well in line with the results of Voelker et al. (2009), but more developmental studies of genotype effects that include environmental moderators are needed to provide an understanding of the patterns and mechanisms of environmental factors and genotype interplay. Given that the COMT gene Val158 Met polymorphism by maternal education interaction disappears during the second wave in the present study, it seems to be restricted to a particular age or skill level. Children’s abilities in autobiographical memory and narrative development undergo major developments during the preschool years (Fivush & Nelson, 2006; Nelson & Fivush, 2004). Children start referencing their past as soon as they start talking (Fivush & Fromhoff, 1988), but ‘real’ autobiographical memory emerges by age 6 (Howe, Courage, & Edison, 2003). Similarly, during the preschool years children’s narrative abilities improve: they provide more details and their stories become more coherent (Fivush et al., 1995; Haden, Haine, & Fivush, 1997). At the age of four these skills are rather rudimentary and children are only beginning to acquire competence in recounting their past experience and telling personal autobiographical narratives. It seems that at the early ages the Met/Met homozygote children of mothers with a lower education may be at an advantage compared to their Val/Met counterparts. Yet, it appears that by the age of six these children too have mastered the basics of these skills, and the persistent differences are not related to the COMT gene Val158 Met polymorphism by maternal education interaction. The present study did not address the valence of the recounted experiences, which could be considered in future studies. There is evidence that the Val158 Met genotype of the COMT gene affects the processing of emotional stimuli (Drabant et al., 2006), which means that the COMT gene Val158 Met polymorphism could affect memories of the positive and negative events. Understanding normal cognitive development and the role of the COMT gene Val158 Met genotype is vital before we can fully capture its place in the development of mental disorders. The article made an important contribution to the literature showing that the COMT gene Val158 Met genotype has an effect on young children’s autobiographical memory and that the effect is modified by environmental factors that stem from the differences in maternal education level. Acknowledgements Research for the article was supported by the Estonian Research Council Institutional Research Funding (IUT 42-2 and IUT 20-40), Personal Post Doctoral Research Funding (PUTJD97), and European Regional Development Fund ERC Program TerVe (ELIKTU 3.2.1002.11-0002). References Barnett, J. H., Heron, J., Ring, S. M., Golding, J., Goldman, D., Xu, K., et al. (2007). Gender-specific effects of the catechol-O-methyltransferase Val108/158Met polymorphism on cognitive function in children. 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