Explaining the curvilinear relationship between age at first birth and depression among women

Social Science & Medicine 72 (2011) 494e503

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Explaining the curvilinear relationship between age at first birth and depression among women Daniel L. Carlson Department of Sociology, The Ohio State University, 238 Townshend Hall, 1885 Neil Ave. Mall, Columbus, OH 43202, USA

a r t i c l e i n f o

a b s t r a c t

Article history: Available online 15 December 2010

Recent research suggests that the effect of age at first birth on mental health for women is curvilinear, with first births at both young (age 20 and younger) and older ages (after age 30) being positively associated with psychological distress. Scholars have theorized that accumulated disadvantages and physical health problems associated with age at first birth explain this pattern, although empirical support for these explanations has varied. Using data from the US National Longitudinal Survey of Youth 1979, this study provides evidence of an alternative explanation for this curvilinear relationship through its focus on: 1) the relationship between deviations from expected age at first birth and women’s actual age at first birth, and 2) the effect deviations from expected age at first birth have on mental health. Results indicate that deviating from their expected age at first birth results in higher levels of depressive symptoms for women in midlife who transition into parenthood both earlier and later than expected. These deviations from expected birth timing account for the upward trend in depressive symptoms at older ages of first birth, but explain only a small amount of the higher levels of depressive symptoms at younger ages. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: USA Depression Mental health Identity Life course Age at first birth Birth timing Women’s health

Introduction Recent studies provide preliminary evidence of a curvilinear relationship between mothers’ age at first birth and depressive symptoms suggesting that entering parenthood at both young (age 20 or younger) and older ages (after age 30) negatively affects mothers’ mental health and that these effects persist throughout the life course (Koropeckyj-Cox, Pienta, & Brown, 2007; Mirowsky & Ross, 2002; Spence, 2008). Scholars have theorized that accumulated disadvantages and physical health problems associated with childbearing at young and older ages explain the curvilinear pattern, although empirical support for these hypotheses has varied. In this study I develop and test an alternative hypothesis: that the curvilinear relationship between age at first birth and depressive symptoms is explained by deviations from one’s expected age at birth and the effect such deviations have on mental health. Substantial research and theory indicates that deviating from life course expectations can result in violations of social age norms (Neugarten, Moore, & Lowe, 1968) and identity discrepancies (Higgins, 1987; Marcussen & Large, 2003) that are, in turn, strongly associated with mental health (Alexander & Higgins, 1993; Higgins, 1987; Koropeckyj-Cox et al., 2007). If first births at young and older ages are mistimed, this may explain the elevated levels of

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psychological distress found at these ages. This would mean that the previously observed relationship between age at first birth and psychological distress is largely spurious, and primarily the result of births at younger and older ages being mistimed and/or nonnormative. Background Age at first birth and psychological distress Births at young ages have significant negative consequences for mental health. Young parents (age 20 and younger) consistently exhibit lower levels of mental health and higher levels of psychological distress throughout life than those who enter parenthood later (Kalil & Kunz, 2002; Mirowsky & Ross, 2002; Spence, 2008). The primary explanation for this general pattern is that births early in the life course produce cumulative disadvantages which, over time, undermine mental health, in part by increasing exposure to stressful life circumstances and leading to further disadvantages (Elder, 1998). Consistent with this perspective, research shows that births at young ages are related to low educational and occupational attainment (McLanahan & Sandefur, 1994), higher risks for permanent singlehood, long-term cohabitation, and marital dissolution (Qian, Lichter, & Mellott, 2005), and poor physical health in later life (Mirowsky, 2002; Ozalp, Tanir, Sener, Yazan, & Keskin, 2003). All of these are known risk factors for

D.L. Carlson / Social Science & Medicine 72 (2011) 494e503

psychological distress (Farmer & Ferraro, 1997; Turner, Wheaton, & Lloyd, 1995). Additionally, age at first birth may also be related to the presence and age of children in the home, and the number of children born to mothers, all of which have also been linked to mental health (Evenson & Simon, 2005; Umberson & Gove, 1989). Findings that all of these factors mediate the association between first births at young ages and mental health in later adulthood (Koropeckyj-Cox et al., 2007; Mirowsky & Ross, 2002; Spence, 2008) provide substantial weight to the cumulative disadvantage hypothesis. Despite evidence to support the cumulative disadvantage hypothesis, the association between first births at young ages and psychological distress may be spurious or a reflection of social selection (Kalil & Kunz, 2002; Mollborn & Morningstar, 2009). As such, any attempt to estimate a causal relationship between age at first birth and psychological distress should account for respondent’s background. Important factors related to age at first birth, such as parents’ socioeconomic status (McLanahan & Sandefur, 1994), childhood family structure (McLanahan & Sandefur, 1994), and mental health in adolescence (Kalil & Kunz, 2002; Kessler et al., 1997; Mollborn & Morningstar, 2009) are also associated with mental health in adulthood (Evans & English, 2002; Kessler et al., 1997). Although many studies attempt to control for these background factors, rarely are they all accounted for. Even so, other factors may be responsible for the association of first births at young ages with mental health. It is therefore important to identify and account for such factors. In this study I identify a possible confounder e deviations from expected first birth timing e and provide a theoretical rationale for why it may account for this association. Despite substantial attention to the mental health consequences of births at young ages, much less is known about the impact of delaying parenthood until older ages. A few recent studies suggest that childbirth at older ages may also be associated with lower levels of mental health. One study, using a nationally representative sample of U.S. adults age 18e95 by Mirowsky and Ross (2002) finds that the optimal age at first birth for women is approximately age 30, with depressive symptoms increasing thereafter. A second study of U.S. women born between 1923 and 1937 by Spence (2008) finds that in addition to entering parenthood at young ages late childbearing, defined as completing childbearing after age 35, is also associated with increased levels of depressive symptoms in later life. Both studies hypothesize that mother’s physical health, in addition to mediating the effects of young ages at first birth, may also explain the upward trend in depressive symptoms at older ages of birth because childbearing at older ages is also associated with health detriments later in life (Alonzo, 2002; Mirowsky, 2002, 2005; Ozalp et al., 2003). Nonetheless, although Mirowsky and Ross (2002) find physical health alone to fully mediate the curvilinear effect of age at first birth among women in their study, Spence (2008) finds that physical health mediates only the effect of young childbearing on depressive symptoms. Several factors such as the operationalization of late childbearing, the age composition of the samples, and the comparison groups used may account for the inconsistent findings regarding the role of physical health. First, Mirowsky and Ross examine the effects of older ages at first birth while Spence examines the effects of completing childbearing at older ages. It is possible that beginning childbearing in one’s thirties may have fundamentally different health consequences than beginning childbearing earlier but completing it at older ages. Second, in contrast to the wide age range in the Mirowsky and Ross sample, the Spence study is limited to a specific cohort of women and focuses exclusively on physical and mental health outcomes in later life (age 65e83). Therefore, the immediate physical health consequences of pregnancy associated

495

with late childbearing may only be captured by the Mirowsky and Ross study while the Spence study may only capture long-term effects. Lastly, Spence examines the effect of young and late childbearing among parents only, while Mirowsky and Ross employ age at first birth as an internal moderator of the difference in depressive symptoms between parents and childless adults. Even if physical health does not explain the effect of late childbearing on the mental health of parents, it may explain why age at first birth structures differences between parents and the childless. Given the nascency of this research, especially as it pertains to childbearing at older ages, it is clear that the association of age at first birth with depressive symptoms warrants greater attention. It is possible that factors other than physical health (for young and older ages at first birth) and the accumulation of social or economic disadvantages (for younger ages at first birth), are implicated in producing the curvilinear relationship between age at first birth and depressive symptoms. Indeed, birth mistiming is a likely confounding factor with the potential to explain this relationship but which, to date, has not been investigated. Mistimed births negatively affect mental health (for review see Gipson, Koenig, & Hindin, 2008; Logan, Holcombe, Manlove, & Ryan, 2007) and substantial evidence indicates that a large proportion of teen births are mistimed (Beck et al., 2002; Kost & Forrest, 1995). If first births at older ages are also mistimed, this may explain the curvilinear association between age at first birth and depressive symptoms. Although research suggests that births at older ages are less likely to be mistimed than first births in adolescence (Beck et al., 2002; Kost & Forrest, 1995), this largely results from the predominant conceptualization of birth mistiming in the literature, where only births occurring earlier than expected are labeled “mistimed” (for review see Gipson et al., 2008; Logan et al., 2007). If we redefine mistiming to mean a birth that does not occur at the age one expected, then first births at older ages are also likely to be mistimed since they likely deviate from expectations. With the proportion of first births occurring after age 30 rising dramatically e in 2006, 1 out of every 12 first births was to a U.S. woman age 35 years and over, compared to 1 out of every 100 in 1970 (Matthews & Hamilton, 2009) e it is quite plausible that many women expected their first birth to occur at younger ages. In sum, this study aims to expand the definition of mistimed births to include those that are both earlier and later than expected, to examine the association between age at first birth and mistiming, and finally, to determine whether mistiming accounts for the curvilinear relationship between age at first birth and psychological distress. At the same time it is necessary to test the mistiming hypothesis proposed in this study against past hypotheses used to explain the relationship between age at first birth and psychological distress with the purpose of examining whether birth mistiming contributes a unique explanation for this relationship. Deviating from timing expectations: off-time births and identity discrepancies Two complementary theoretical perspectives, the normative life course perspective and self-discrepancy theory, offer insight into the processes through which mistimed births, both early and late, pose problems for mental health. The normative life course perspective (Neugarten et al., 1968) suggests that societal expectations for age-appropriate behavior include a prescriptive timetable for the timing of major life events, such as the transition into parenthood. Accordingly, “individuals develop a concept of the ‘normal expectable life cycle,’ a set of anticipations that certain life events will occur at certain times, and a mental clock telling them whether they are on time or off time”

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(Neugarten, 1979). People subsequently use these internal social clocks to assess their personal development. Because transitions into new life stages constitute major turning points that produce changes in self-concept and identity, unexpected, off-time events often present life crises which may negatively affect well-being (Neugarten, 1979). Self-discrepancy theory helps explain why off-time transitions into parenthood and related adjustments to self-concept and identity may result in crisis by noting that a person’s ability to meet expectations for identity plays an important role in shaping mental health outcomes. According to developmental psychologists, identity is an amalgamation not only of a person’s current social structural positions (i.e., roles and statuses) but also the person’s expectations for future roles and statuses (Markus & Nurius, 1986). At the heart of self-discrepancy theory is the premise that discrepancies between anticipated identities and actual identities produce psychological distress because they result in: 1) threat to, or loss of, identity, 2) negative feelings about one’s current and future self, and 3) unanticipated and undesirable adjustments to identity (Higgins, 1987; Marcussen & Large, 2003). Indeed, identity discrepancies related to the performance of, and strains associated with, parenting (Cast, 2004; Simon, 1992) and becoming a parent (Alexander & Higgins, 1993; Reich, Harber, & Siegel, 2008) significantly and negatively affect parents’ mental health. Although distress related to changes in self-concept and identity constitute one potential pathway through which off-time transitions into parenthood may negatively affect mental health, there are several other mechanisms, according to the normative life course perspective, through which the off-time transition into parenthood may threaten psychological well-being. First, it may deprive one of peer support and result in social isolation (Brim & Ryff, 1980). Second, it may deprive one of the satisfaction and meaning, and thus psychological benefit, an on-time transition into parenthood could provide (Lazarus & Folkman, 1984; Thoits, 1983). Thirdly, an off-time transition may leave one ill-prepared for the parental role (Hagestad, 1986). Lastly, off-time transitions into parenthood may precipitate negative social sanctions from one’s community and/or social network (Neugarten et al., 1968) which could also damage well-being. Although these processes are likely similar for both very young and old ages at first birth, potential differences may exist. Unlike young parents, those who become parents at older ages are likely well-prepared for parenthood having expected it for some time. Furthermore, those with first births at older ages are potentially more likely to have peers with children and thus be less socially isolated. Nevertheless, some degree of social isolation is still likely, especially if one’s peers’ children are at different developmental stages than their own. One’s personal expectation for age at first birth is likely fundamentally shaped by social norms related to the transition into parenthood (Settersten & Hagestad, 1996). Whether transitions into parenthood at young and older ages result in an identity discrepancy, social isolation, loss of meaning derived from parenthood, decreased satisfaction with parenthood, and/or social sanction, it is clear that young and older ages at first birth are likely problematic for a mother’s mental health because they are off-time. That both earlier and later than expected transitions into parenthood likely negatively affect mental health suggests that the effect of deviations from timing expectations on psychological distress is curvilinear. The lowest levels of psychological distress are most likely found when women meet their expected timing and as they deviate (either early or late) from expectations, psychological distress should increase as deviations grow larger. Therefore, the reason first births at young and older ages are likely related to increased distress is because they are mistimed and/or non-normative.

Method Data for this study come from the National Longitudinal Survey of Youth 1979 (NLSY79), a nationally representative sample of 12,686 U.S. male and female youths age 14e22 in 1979. Each respondent was interviewed annually from 1979 to 1994 and biennially since. At first interview, respondents were asked a set of questions about their expectations for the timing of parenthood. Beginning in 1998, respondents who reached age 40 completed a one-time health module containing a seven-item portion of the Center for Epidemiological Studies Depression scale (CES-D). For the purposes of this study, I restrict analysis to mothers who expected children but who were childless in 1979 and not expecting a child within that year. As of 2006, all original NLSY79 female respondents had turned age 40 and were eligible to take the Health Module questionnaire. 68.4% (n ¼ 4296) of the original 6283 female respondents completed it. Due to funding constraints, the original NLSY sample of 6283 female youths was reduced to 4952 by 1990 when most military respondents and the supplemental subsample of economically disadvantaged non-black, non-Hispanic respondents were dropped. Of the remaining 4952 female respondents eligible for interview, 86.8% (n ¼ 4296) had completed at least some portion of the NLSY79 Health Module by 2006 and were therefore eligible for inclusion in this study. Logistic regression analysis (not shown) on the odds of a respondent being lost to attrition for the 4952 eligible respondents showed that non-African-Americans and older respondents in 1979 were more likely to be lost to attrition than African-Americans and younger respondents. Results suggested no difference in expectation for parenthood or expected age at first birth. Of the 4296 female respondents who completed the Health Module by 2006, 76.2% (n ¼ 3273) met criteria for inclusion in the sample. Of excluded respondents, 690 were expecting or already had their first child prior to the 1979 interview, 291 reported no expectation for children, and 42 did not provide a valid response. Of those meeting criteria for inclusion, 3071 (93.8%) reported on their expectations for the timing of parenthood. In all, 2699 of respondents who expected to have at least one child had become mothers by age 40. In some cases, information on respondent’s age at first birth was missing (n ¼ 198), these cases were excluded from analysis. The final analytic sample consists of N ¼ 2501 cases. Dependent variable Depressive symptoms is the dependent variable and is measured with a 7-item version of the CES-D at age 40.1 Scores on individual items range from 0 to 3 with higher scores indicating more frequent symptoms. Responses were mean scaled (alpha ¼ .83) which required five out of seven valid responses, summed, and then logged [ln(depression score þ 1)] to adjust for skew. Independent variables Age at first birth and Age at first birth squared is a continuous variable which I standardized (z-score) to avoid collinearity. Offtime deviation of first birth and Off-time deviation of first birth squared assesses birth mistiming and is measured as the difference

1 The NLSY79 Health Module includes the following seven questions from the CES-D: “During the past week . (1) I did not feel like eating; my appetite was poor; (2) I had trouble keeping my mind on what I was doing; (3) I felt depressed; (4) I felt that everything I did was an effort; (5) my sleep was restless; (6) I felt sad; (7) I could not get “going”.

D.L. Carlson / Social Science & Medicine 72 (2011) 494e503

between respondent’s actual and expected age at first birth, as reported in 1979. Positive values on this variable indicate late first births, negative values indicate early first births, and a value of 0 indicates first births that occurred at the age the respondent expected (Range ¼ 20 to þ19). It is important to note that because birth timing expectations are measured only at baseline, potential changes in expectations over time cannot be addressed. Research on fertility suggests significant stability in parity expectations over time (Hayford, 2009; Rindfuss, Morgan, & Swicegood, 1988) but no studies have directly examined within-individual change in timing expectations. In fact, the measurement of expectations early in the life course means that they are assessed prior to birthdan improvement over previous studies (for review see Gipson et al., 2008; Logan et al., 2007). Measuring intentions following birth is problematic because respondents’ recollections of their intentions may be inaccurate, biased, and subject to revision following births. Furthermore, changes in expectations, if they occur, are often a way of coping with the stress of not meeting them (Pearlin & Skaff, 1996). Thus, even if expectations change, the experience of stress and, in turn, psychological distress, may continue to be tied to earlier expectations. Nevertheless, changes to one’s expectations may be problematic especially if they result in an overestimation of the effect of timing deviations on mental health. This could occur if actual timing deviations are systematically larger than what is observed. Expectations, if they change, are most likely to change for two groups e those who delay childbirth and those who were youngest when expectations were assessed. If expectations change, timing deviations for those who give birth at older ages are likely to be smaller than what is reported in this study as respondents upwardly revise their timing expectations as they age. In this case, estimates of the effect of timing deviations are likely to be conservative. For those who were youngest when expectations were assessed in 1979 concerns are two-fold. First, younger respondents’ expectations may not be as well formulated and accurate as older respondents’ and second by virtue of their younger ages they have a larger range of possible birth ages and potentially more time to revise expectations. Supplemental analyses (not shown) indicated that although off-time deviations did vary across age in 1979 (p < .01), younger respondents in 1979 did not exhibit greater average deviations than older respondents. However, the results suggested much more variability in timing deviations and expected ages at first birth for younger respondents than older ones. The odds of experiencing both earlier (p < .001) and later than expected first births (p < .01) compared to an ontime first birth declined as age at first interview in 1979 increased. This suggests that younger respondents were more likely to both over and underestimate their eventual age at first birth. A Physical health scale (alpha ¼ .80) was used to assess mother’s physical health at age 40 and to examine whether physical health mediated the association between age at first birth and depressive symptoms. The scale consists of responses to five questions: (1) In general, how would you say your health is? (2) Does your health limit you in moderate activities, such as moving a table, pushing a vacuum cleaner, bowling or playing golf? (3) Does your health limit you in climbing stairs? (4) During the past 4 weeks, how much did pain interfere with your normal work? (5) During the past 4 weeks, how often did you have a lot of energy? Question 1 ranges from 0 (poor) to 5 (excellent), questions 2 and 3 are coded (1) Yes, limited a lot; (2) Yes, limited a little, (3) No, not limited at all. Question 4 ranges from (1) not at all to (5) extremely. Finally, question 5 ranges from (1) all of the time to (6) none of the time. Each item was mean centered to provide equal weight to each, and summed. Lastly, I multiplied the scale by 1 to reorient it so that higher values indicate better physical health.

497

To test whether cumulative disadvantages explained the association between age at first birth and depressive symptoms, I included measures for socioeconomic status and family formation at age 40. Socioeconomic status was approximated using Education (in years), Logged household income, and a continuous measure of hours worked per week. Measures for family formation include current marital status, which is categorized as: Currently in 1st marriage (reference category), Never married, Transitioned out of marriage (widowed, divorced, and separated), and Remarried, the number of children ever born as of age 40, a dummy variable for child present in household at age 40, and the age of the youngest child in the home, measured in years, which is tested as an internal moderator of having children present in the household. Control variables included: Race/ethnicity (reference ¼ nonHispanic white), Respondent’s age in 1979, poverty status in 1978e1979 (1 ¼ in poverty), and respondents’ household family structure at age 14 e both biological parents (reference), single mother, mother-stepfather, and other family structure. To control for respondents’ mental health before birth, models incorporated a retrospective measure (taken at age 40) of whether the respondent had ever been diagnosed with a mental illness prior to the 1979 interview date (1 ¼ yes) and Rotter’s locus of control scale which was assessed in 1979 (alpha ¼ .38) and is commonly used to control for baseline mental health in NLSY79 studies (Kalil & Kunz, 2002; Reynolds & Baird, 2010). Lastly, because it is possible that older ages at first birth may reflect fertility problems, which may be negatively related to mental health (Greil, 1997) and account for the association between older ages at first birth and depressive symptoms, I controlled for the number of miscarriages/stillbirths experienced as of age 40. Analytic strategy Descriptive statistics determined whether births at young ages (age 20 or younger) and older ages (after age 30) were generally mistimed. Second, using OLS regression, I examined the association between age at first birth and depressive symptoms along with variables that may explain this association. I conducted the analysis in several stages. First, I examined the basic association between age at first birth and depressive symptoms, net of controls for mothers’ background characteristics. In the second and third step, I examined the degree to which physical health and cumulative disadvantages (as represented by family formation and socioeconomic status) explained this association. In the fourth step, I assessed whether deviations from expected timing of first birth accounted for the association. Finally, I estimated a full model with all explanatory factors included. Although the degree of mediation or spuriousness in the curvilinear association between age at first birth and depressive symptoms can be assessed by measuring the change in the coefficients for age at first birth after explanatory factors are added to the model, this approach does not necessarily provide a clear indication of whether these factors account for increases in depressive symptoms at young ages of first birth, older ages at first birth, or both. In short, it can be difficult to surmise how each of these factors affects the curvilinear association simply by examining changes in coefficients. Therefore, in the last set of models I created a categorical, ordinal measure for age at first birth and examined the degree to which the inclusion of each of the explanatory factors accounted for differences in depressive symptoms between those whose age at first birth was associated with the lowest predicted levels of depressive symptoms and those who had their first child at the youngest and oldest ages. Results Tables 1 and 2 display descriptive statistics for the sample of mothers who expected children. On average, women in the sample

D.L. Carlson / Social Science & Medicine 72 (2011) 494e503

Variables

Mean (s. d.)

Depressive symptoms (logged) Age at first birth (years) Age at first birth squared Off-time deviation of first birth (years) Off-time deviation of first birth squared Physical health scale e age 40 Education age 40 (years) Income age 40 (logged) Work hours age 40 Never married age 40 Transitioned out of marriage age 40 Remarried age 40 Number of children born as of age 40 Child present in household e age 40 Age of youngest child in home African-American Hispanic Other race/ethnicity Age of respondent (1979) Poverty status ’78e’79 Family structure age 14 e single mother Family structure age 14 e mothere stepfather Family structure age 14 e other Number of miscarriages/stillbirths as of age 40 Ever diagnosed mentally ill (1979) Rotter locus of control scale (1979)

1.14 (0.92) 24.80 (5.30) 643.14 (279.93) 1.50 (5.24) 29.64 (49.04) 0.18 (3.80) 13.51 (2.30) 10.51 (1.81) 28.63 (19.99) 0.10 0.26 0.16 2.29 (1.09) 0.93 10.19 (4.97) 0.25 0.20 0.07 17.29 (2.16) 0.24 0.16

Range Low

High

0 14 196 20 0 16.46 0 0 0

4.16 42 176 19 40 3.78 20 13.08 168

1

10

1

24

14

22

0.08 0.34 (0.83)

0

6

0.00 7.23 (2.40)

0

12

0.07

had their first child just before turning 25 years of age, which was 1.5 years later than expected. These averages, however, mask a great deal of heterogeneity in age at first birth, timing expectations, and deviations from timing expectations. As shown in Table 2, there was a great deal of variation in age at first birth. 25.1% of the sample had their first child before age 21, while 16.3% had their first child after age 30. The most common time period for first births was between ages 21 and 25. Noted in the table is the significant variation in expected age at first birth, which is somewhat associated with actual age at first birth. Women who had their first child at the youngest ages expected to become mothers earlier than other women, at approximately age 22. Conversely, women who entered parenthood after age 30 expected to enter parenthood later than all other women, albeit only slightly later than the next oldest birth category (first birth between ages of 26 and 30) and only 2.4 years later than women who gave birth before age 21. As noted by the average off-time deviations for these age groups, women who entered parenthood after age 30 exhibited the largest average off-time deviation e more than 9 years later than expected. Indeed, only one-tenth of one percent of women who had their first child after age 30 did so within one year of their expected

age at first birth. This provides compelling evidence that entry into parenthood after age 30 is generally a mistimed transition for women. In contrast, parenthood before age 21 occurs on time far more frequently. Although births to young mothers occurred 3.3 years earlier than expected on average, nearly 30% of these mothers had their first child within 1 year of their expected timing. Table 3 presents results from OLS regression analyses of depressive symptoms on age at first birth and off-time deviation of first birth. Results from Model 1 showing the effect of age at first birth with controls for respondents’ background characteristics, confirmed findings from past studies of a curvilinear relationship between a woman’s age at first birth and depressive symptoms (see Fig. 1 where the standardized coefficients are transformed back into years). As seen in Fig. 1, increases in age at first birth resulted in lower levels of depressive symptoms for women until age at first birth was between age 31 (y ¼ .9965) and 32 (y ¼ .9963). After age 32, increases in age at first birth resulted in increases in depressive symptoms. Mother’s physical health in Model 2 somewhat mediated the curvilinear relationship between age at first birth and depressive symptoms, accounting for approximately 10.2% (.0490  .0440/ .0490 ¼ .1020) of the curvilinear effect. Results from Model 3 which include measures of family formation and socioeconomic status indicated that although these factors mediated a significant portion (.0490  .0335/.0490 ¼ 31.6%) of the curvilinear relationship between age at first birth and depressive symptoms, it remained significant (p < .05). Model 4 tested whether deviations from expected first birth timing explain the effect of age at first birth on depressive symptoms. The inclusion of measures for off-time deviation of first birth resulted in a 57.3% reduction (.0490e.0209) in the coefficient for the age at first birth quadratic term, reducing the coefficient to nonsignificance. Variance-inflation Factor and Tolerance coefficients (not shown) provided no evidence of multicollinearity in these models. The absence of multicollinearity was further evidenced by the lack of substantive changes in the standard errors of the age at first birth coefficients. Although off-time deviations mediated the curvilinear relationship between age at first birth and depressive symptoms, a significant linear effect of age at first birth remained in Model 4. When all variables were included in the models only a marginally significant (p < .10) negative association between age at first birth and depressive symptoms remained. Combined, these three sets of factors accounted for 76.5% (.0490  .0115/ .0490 ¼ .7653) of the curvilinear effect. As shown in Table 3, the effect of off-time deviations on depressive symptoms is curvilinear and U-shaped. Although the arc of the curve was somewhat reduced in magnitude (.0013e.0010) when family formation, socioeconomic status, and physical health were controlled (Model 5), the coefficient for the squared term remained significant at the p < .05 level. Fig. 2 shows the relationship between off-time deviation of first birth and depressive symptoms. The lowest levels of depressive symptoms were found when mothers met their expected timing for entry into parenthood. ˇ

Table 1 Descriptive statistics e means, proportions, and standard deviations (in parentheses) for all variables (n ¼ 2501).

ˇ

498

Table 2 Means and standard deviations for mothers’ expected age at first birth and off-time deviation of first birth by age at first birth (n ¼ 2501). Age at first birth

N (% of sample)

Expected age at first birth

Off-time deviation of first birth

Mean

Std. dev

Mean

Std. dev

N (% of category)

Before age 21 Age 21e25 Age 26e30 After age 30 Total

636 833 625 407 2501

21.91 23.19 24.21 24.29 23.30***

2.84 2.71 2.90 2.92 2.97

3.33 0.25 3.53 9.35 1.50***

2.97 2.87 3.15 3.65 5.24

187 392 104 4 687

(25.4%) (33.3%) (25.0%) (16.3%) (100.0%)

Note: ***p < .001 (analysis of variance e ANOVA).

First births within 1 year of expected age

(29.4%) (47.1%) (16.6%) (0.1%) (27.5%)

D.L. Carlson / Social Science & Medicine 72 (2011) 494e503

499

Table 3 OLS regression of logged depressive symptoms on age at first birth, health, deviations from timing expectations, and sociodemographic characteristics of mothers (n ¼ 2501).

Intercept Age at first birth (standardized e z-score) Age at first birth squared (standardized e z-score) Physical health scale e age 40 Education e age 40 Income e age 40 (logged) Work hours e age 40 Never married e age 40 Transitioned out of marriage e age 40 Remarried e age 40 Number of children born as of age 40 Child present in household e age 40 Child present in household  age of youngest child in home Off-time deviation of first birth Off-time deviation of first birth squared African-American Hispanic Other race/ethnicity Age of respondent (1979) Poverty status ’78e’79 Family structure age 14 e single mother Family structure age 14 e mother-stepfather Family structure age 14 e other Ever diagnosed mentally ill (1979) Rotter locus of control scale (1979) Number of miscarriages/stillbirths as of age 40 R2

Model 1

Model 2

Model 3

Model 4

Model 5

B (s.e.)

B (s.e.)

B (s.e.)

B (s.e.)

B (s.e.)

1.14*** (0.16) 0.12*** (0.02) 0.05** (0.02)

1.05*** (0.15) 0.08*** (0.02) 0.04** (0.02)

2.03*** (0.23) 0.08* (0.03) 0.03* (0.02)

1.10*** (0.17) 0.19*** (0.04) 0.02 (0.02)

1.49*** (0.23) 0.07y (0.04) 0.01 (0.02)

e e e e e e e e e e e e e e e e e e e e e e e e

0.10*** (0.00) e e e e e e e e e e e e e e e e e e e e e e

e e 0.02* (0.01) 0.04*** (0.01) 0.00*** (0.00) 0.14* (0.07) 0.21*** (0.05) 0.08 (0.05) 0.02 (0.02) 0.12 (0.11) 0.00 (0.01) e e e e

e e e e e e e e e e e e e e e e e e e e 0.01 (0.01) 0.00* (0.00)

0.10*** (0.00) 0.01 (0.01) 0.03*** (0.01) 0.00 (0.00) 0.07 (0.06) 0.12** (0.04) 0.07 (0.05) 0.01 (0.02) 0.00 (0.10) 0.00 (0.00) 0.00 (0.01) 0.00* (0.00)

0.02 (0.05) 0.11* (0.05) 0.05 (0.07) 0.00 (0.01) 0.08 (0.05) 0.02 (0.05) 0.06 (0.07) 0.08 (0.07) 0.88*** (0.27) 0.02** (0.01) 0.13*** (0.02) 0.05

0.01 (0.04) 0.09* (0.05) 0.05 (0.07) 0.01 (0.01) 0.02 (0.04) 0.02 (0.05) 0.03 (0.07) 0.03 (0.06) 0.41 (0.26) 0.01* (0.01) 0.07** (0.02) 0.21

0.02 (0.05) 0.15** (0.05) 0.04 (0.07) 0.00 (0.01) 0.03 (0.05) 0.00 (0.05) 0.03 (0.07) 0.05 (0.07) 0.79** (0.27) 0.02* (0.01) 0.12*** (0.02) 0.08

0.01 (0.05) 0.11* (0.05) 0.05 (0.07) 0.01 (0.01) 0.08 (0.05) 0.02 (0.05) 0.05 (0.07) 0.08 (0.07) 0.88*** (0.27) 0.02** (0.01) 0.13*** (0.02) 0.05

0.04 (0.05) 0.12** (0.05) 0.04 (0.07) 0.01 (0.01) 0.00 (0.04) 0.01 (0.05) 0.00 (0.07) 0.00 (0.06) 0.40 (0.26) 0.01 (0.01) 0.07** (0.02) 0.22

*** p < .001; **p < .01; *p < .05; yp < .10 (two-tailed).

As they deviated, either early or later from their expected timing, depressive symptoms increased. Given that women entering parenthood after age 30 deviated greatly, on average, from their expectations and the effect such deviations have on depressive symptoms, the results indicate that the mental health effects of offtime deviations explain the upward trend in depressive symptoms associated with first births at older ages. This suggests that the association of older ages at first birth with depression is largely spurious, while the effect of young ages at first birth on depressive symptoms is perhaps only partly so. To provide a clearer picture of how physical health, socioeconomic status, family formation, and birth mistiming account for higher levels of depressive symptoms at younger and older ages of first birth,

age at first birth was divided into four categories in Table 4: (1) before age 21, (2) between age 21 and 30, (3) between age 31 and 32 (reference), and (4) after age 32. First births between age 31 and 32 was the reference category based on findings from Table 3 which showed the lowest predicted levels of depressive symptoms at these ages. To remain consistent with past studies, I divided first births younger than age 31 into two categories to highlight levels of depressive symptoms for those with first births at the youngest ages (i.e., before age 21). As shown in Model 1 of Table 4, net of controls for mothers’ background, first births that occurred before age 21 (p < .001), between age 21 and 30 (p < .01), and after age 32 (p < .10) were all associated with higher levels of depressive symptoms compared to first births between the ages of 31 and 32. As seen in

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1.6

1.5

Depressive Symptoms (logged)

1.4

1.3

1.2

1.1

1

0.9

0.8 14

16

18

20

22

24

26

28

30

32

34

36

38

40

Age at First Birth Fig. 1. Depressive symptoms (logged) of mothers by age at first birth.

Models 2 through 4, all 3 sets of factors reduced the difference in depressive symptoms between births before age 21 and births between age 31 and 32. As seen in Model 4, the inclusion of off-time deviations accounted for only a minor portion of the difference

Depressive Symptoms (logged)

1.5

1.25

1

-20 -18 -16 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 16 18 20

0.75

earlier than expect ed first birt hs

on time first births

later than expect ed first births

Deviations from Expected Timing Fig. 2. Depressive symptoms (logged) of mothers by deviations from expected timing of first birth.

(.3596  .3189/.3596 ¼ 11.3%), suggesting that higher levels of depressive symptoms found at very young ages of first birth were only slightly attributable to birth mistiming. Of the 3 explanatory factors, family formation and socioeconomic status explained the largest share (.3596  .2126/.3596 ¼ 40.9%) of the association, although mothers’ physical health explained a substantial portion (33.6%) as well. When all 3 explanatory factors were considered together in Model 5 the difference became non-significant. Both physical health and off-time deviations reduced the difference in depressive symptoms between first births after age 32 and first births between age 31 and 32. Off-time deviations accounted for 16.2% of the difference in depressive symptoms while physical health accounted for 6.8%. The inclusion of off-time deviations alone reduced the effect of older ages at first birth to nonsignificance, again suggesting that the association was largely spurious and due to the fact that these births were mistimed. Interestingly, family formation and socioeconomic status, as noted in Model 3, somewhat suppressed the effect of older ages at first birth on depressive symptoms, something not readily apparent in the results from Table 4. When family formation and socioeconomic status were controlled, a stronger association of older ages at first birth with depressive symptoms emerged. It appears that the positive association of older ages at first birth with depressive symptoms is confounded by the negative association of family formation and socioeconomic status with depressive symptoms and the positive association these factors have with older ages at first birth. It appears then that the mental health benefits of marriage and high socioeconomic status e known correlates of older ages at first birth e may partially ameliorate the negative mental health effects of first births at these ages. Discussion The analyses provide substantial support for the central hypothesis that mistiming of first births, both early and late, account for the curvilinear relationship between age at first birth and psychological distress. Although women’s physical health, family formation, and socioeconomic status had some effect on the curvilinear relationship between age at first birth and depressive symptoms, the inclusion of measures for off-time deviations of first birth reduced the curvilinear effect of age at first birth to non-significance. Complementary analyses where age at first birth was measured categorically were consistent with these results and indicated that although off-time deviations from first birth timing accounted for some of the increase in depressive symptoms at the young ages of first birth, the effect of young ages at first birth remained significant when off-time deviations were included in the models. Conversely, the inclusion of offtime deviations reduced the association for older ages at first birth to non-significance. As hypothesized, the association between older ages at first birth and depressive symptoms was found to be spurious. The results concerning first births at young ages indicate only a small degree of spuriousness. Consistent with the normative life course perspective and selfdiscrepancy theory depressive symptoms were lowest when women met their expectations for age at first birth and increased as they deviated, either early or late, from expectations. These results are consistent with the interpretation that expectations for identity, the timing of events in life course, and the ability to meet these expectations are important for psychological well-being. That the effect of off-time deviations from expected birth timing persisted and changed little after accounting for other explanatory variables further supports this conclusion. What this study provides, that previous research on the mental health implications of age at first birth has not, is a focus on women’s subjective assessment of their personal development and

D.L. Carlson / Social Science & Medicine 72 (2011) 494e503

501

Table 4 OLS regression of logged depressive symptoms on categorical measure of age at first birth, physical health, family formation and socioeconomic status, and deviations from timing expectations for mothers (n ¼ 2501). Model 1

Model 2

Model 3

Model 4

Model 5

Background characteristics and controls

Model 1 þ physical health

Model 1 þ family formation and socioeconomic status

Model 1 þ off-time deviations

All variables

B (s.e.)

B (s.e.)

B (s.e.)

B (s.e.)

B (s.e.)

Intercept

1.03 (.18)

0.98 (0.16)

1.98 (0.24)

1.02 (0.19)

1.48 (0.24)

First birth before age 21a

0.36*** (0.08)

0.24*** (0.07)

0.21* (0.09)

0.32*** (0.10)

0.15 (0.10)

First birth age 21e30a

0.19** (0.07)

0.14* (0.07)

0.14y (0.08)

0.18* (0.08)

0.13y (0.08)

First birth after age 32a

0.17y (0.09)

0.15y (0.08)

0.20* (0.09)

0.14 (0.10)

0.12 (0.09)

R2

0.04

0.21

0.08

0.04

0.22

***p < 0.001; **p < 0.01; *p < 0.05; yp < 0.10 (two-tailed). Note: background characteristics include all variables, other than age at first birth, in Model 1 of Table 3. a Reference ¼ first birth between age 31 and 32.

the role such assessments play in shaping mental health outcomes. Although this paper is not the first to examine the role of timing expectations for mental health, my approach in redefining birth mistiming to include not only earlier than expected births but also later than expected births greatly expands our knowledge regarding the mental health consequences of birth intentionality. Because previous research on birth intentions and mistiming have focused solely on the mental health consequences of unwanted or earlier than expected births, they have ignored the possibility that later than expected births also pose threats to mental health. This is clearly problematic as the results of this study show since later than expected births also affect mental health. Not only are later than expected births problematic for mental health, they are just as damaging as earlier than expected births. That this is the case should not surprise since many similarities exist between earlier and later than expected births. In both cases, goals for identity may be threatened and social norms for entry into parenthood violated. The hypothesis that birth mistiming would explain higher levels of distress at both young and older ages at first birth rested on the premise that births at these ages would be generally mistimed. In my examination of the association between age at first birth and birth mistiming I found that although less than one percent of women who had their first child after age 30 were within 1 year of their expected age at first birth, nearly 30% of women who had their first child prior to age 21 met their timing expectations. Despite being consistent with previous findings of high levels of mistiming among teen mothers, this non-trivial percent of on-time births likely explains why mistiming accounts for only a very small amount of the mental health effect of young first births. Furthermore, despite the majority of young mothers deviating from expectations, the average deviation was quite small compared to that of older first time mothers whose entry into parenthood was grossly mistimed. Given that the degree of mistiming matters, with increases in deviations resulting in more depressive symptoms, this may also explain why off-time deviations accounted for only a minor portion of the association of young ages at first birth with depressive symptoms. Overall, the results concerning first births at young ages support extant research in that most of the association with depressive symptoms was attributable to socioeconomic status, family formation outcomes, and poor physical health net of a respondent’s background. Consistent with Spence (2008) the results of this study suggest that physical health is a primary

mediator of the mental health effects of first births at young ages. The results also support the findings of Mirowsky and Ross (2002) in that physical health accounted for some of the variation in curvilinear association of age at first birth with depressive symptoms. My findings, however, suggest that physical health plays a much smaller role than noted by Mirowsky and Ross. Nonetheless, this difference in findings may be attributable to the sample characteristics and analytic strategy of this study. First, my sample was limited to 40 year old mothers while the Mirowsky and Ross sample included the full range of adult ages. I examined mental health at age 40 only and was unable to assess the mental and physical health effects of first births after this age. Research on the relationship between age at first birth and physical health indicates that health problems are most pronounced after age 40 (Mirowsky, 2005). Therefore, physical health may greatly contribute to increases in depressive symptoms for women who give birth after age 40, but such effects would not be identified here. It is also possible that the physical health consequences of older childbearing have diminished for recent cohorts. Differences in analytic strategy may also account for the incongruent findings. In this study I examined the effect of age at first birth on the mental health of mothers not the internal moderating effects of age at first birth on mental health differences between parents and childless adults. Furthermore, the Mirowsky and Ross study differs from my own in that they (1) examined potential mediators of age at first birth among both men and women, and (2) employed a quadratic term for age at first birth as an internal moderator of the effect of parenthood for women only. All of these differences not only affect comparison groups and the meanings of coefficients, but they may also affect the values of coefficients, their standard errors, and their statistical significance. Limitations Like all studies, there are a few limitations of this research. First, the sample used in this study represents a specific cohort of U.S. women. Therefore, it cannot be known whether these findings are applicable to all U.S. women, especially regarding the effects of timing deviations on psychological distress and the degree to which timing deviations account for the association between age at first birth and depressive symptoms. Although delayed childbirth may be something that earlier cohorts did not expect, more recent cohorts may expect first births after age 30 with more regularity.

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Although the present study included controls for baseline mental health, one measure was retrospective and did not refer specifically to whether respondents had been diagnosed with clinical depression. This measure is limited in that respondents’ recollection of the timing of their diagnosis may be imperfect and because not everyone with a mental illness receives professional treatment. A locus of control measure was also included, and although a sense of control is correlated with mental health, it is not an indicator of depressive symptoms. Therefore, differential selection into mistimed childbearing on the basis of prior mental health may not be adequately addressed. This study would have benefited from updated and more comprehensive measures of timing expectations. Because measures of expectations were available only at baseline, it is difficult to know whether and to what degree expectations changed over time and the effect changes may have on the relationship between timing deviations and mental health. Although concerns about the validity of expectations are warranted, especially if changes in expectations are likely to affect the associations reported, this study represents an important first step in examining the effect of birth mistiming on the association between age at first birth and mental health. As data become available, future research should address the potential role changes to timing expectations may have on the associations reported here. Lastly, it is important to note that this study was restricted to women who expected to have children. Although the number of women who report absolutely no expectation for children was small examining whether the association between age at first birth and depressive symptoms is also curvilinear for this group has important implications for my conclusion that the curvilinear association of age at first birth with depressive symptoms is due to deviations from timing expectations. Results from supplemental analyses (not shown) on this small subsample indicated no evidence of curvilinearity. Rather, depressive symptoms decreased as age at first birth increased. Although the results of this analysis support the conclusions of this study, these findings should be interpreted cautiously due to the small sample size. Conclusion Researchers know that mistimed births are problematic for mental health. Yet, the mental health consequences of birth mistiming had never been considered as a possible explanation for the association of age at first birth with mental health. The results of this study suggest that a primary reason young and older ages at first birth among women are related to higher levels of depressive symptoms is that they are likely to be off-time, and this is especially true for first births that occur at older ages. Acknowledgments An earlier version of this manuscript was presented at the American Sociological Association (ASA) Meetings and received the 2010 Graduate Student Paper Award from the ASA section on Aging and the Life Course. The author would like to thank Kristi Williams and the anonymous reviewers for their helpful comments in producing this article. References Alexander, M. J., & Higgins, E. T. (1993). Emotional trade-offs of becoming a parent: how social roles influence self-discrepancy effects. Journal of Personality and Social Psychology, 65(6), 1259e1269. Alonzo, A. (2002). Long-term health consequences of delayed childbirth: NHANES III. Women’s Health Issues, 12(1), 37e45.

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