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In spite of long-held beliefs that traits related to reproductive success tend to become fixed by evolution with little or no genetic variation, there is now considerable evidence that the natural variation of fertility within populations is genetically influenced and that a portion of that influence is related to the motivational precursors to fertility. We conduct a two-stage analysis to examine these inferences in a time-ordered multivariate context. First, using data from the National Longitudinal Survey of Youth, 1979, and LISREL analysis, we develop a structural equation model in which five hypothesized motivational precursors to fertility, measured in 1979-1982, predict both a child-timing and a child-number outcome, measured in 2002. Second, having chosen two time-ordered sequences of six variables from the SEM to represent our phenotypic models, we use Mx to conduct both univariate and multivariate behavioral genetic analyses with the selected variables. Our results indicate that one or more genes acting within a gene network have additive effects that operate through child-number desires to affect both the timing of the next child born and the final number of children born, that one or more genes acting through a separate network may have additive effects operating through gender role attitudes to produce downstream effects on the two fertility outcomes, and that no genetic variance is associated with either child-timing intentions or educational intentions. [ABSTRACT FROM AUTHOR]

It is necessary to determine if causal influences on developing antisocial behavior change with age to guide both research and theory on its origins. The extent to which the same genetic factors influence antisocial behavior across 4–17 years of age was estimated using 2,482 sibling pairs of varying genetic relatedness. Assessments of antisocial behavior by mothers (4–9 years), mothers and youth (10–13 years), and youth (14–17 years) reflected the changing validity of informants across development. Genetic influences on antisocial behavior at 14–17 years were entirely shared with those on antisocial behavior at 10–13 years according to both informants. Genetic influences on antisocial behavior at 14–17 years were distinct from those at 4–9 years, however. These age differences in genetic influences cannot be fully distinguished from informant differences across age, but the present findings indicate that youth reported to be persistently antisocial during childhood and adolescence are influenced by one set of genetic factors influencing parent-report conduct problems in childhood and a second set of genetic influences on youth-reported delinquency that come into play around the time of the pubertal transition. [ABSTRACT FROM AUTHOR]

Inattentive-hyperactive and oppositional behavior have been hypothesized to be developmental precursors to conduct problems. We tested these hypotheses using a longitudinal sample of 6,466 offspring of women selected from nationally representative US households. Conduct problems across 8-13 years were robustly predicted by conduct problems at 4-7 years, but also were independently predicted to a small extent by both inattentive-hyperactive and oppositional behaviors at 4-7 years. Longitudinal multivariate behavior genetic analyses revealed that the genetic and environmental factors that influence conduct problems at both 4-7 and 8-13 years also influence the putative precursors at 4-7 years. After genetic and environmental influences on conduct problems at 4-7 years were taken into account, however, inattentive-hyperactive and oppositional behavior at 4-7 years shared causal influences with conduct problems 8-13 years to a negligible extent. These findings suggest that after early conduct problems are controlled, little is gained in terms of prediction or understanding genetic and environmental influences on later child conduct problems by treating early inattentive-hyperactive and oppositional behavior as developmental precursors to later conduct problems. [ABSTRACT FROM AUTHOR]

Predictive associations between parenting and temperament during the first year of life and child conduct problems were assessed longitudinally in 1,863 offspring of a representative sample of women. Maternal ratings of infant fussiness, activity level, predictability, and positive affect each independently predicted maternal ratings of conduct problems during ages 4-13 years. Furthermore, a significant interaction indicated that infants who were both low in fussiness and high in predictability were at very low risk for future conduct problems. Fussiness was a stronger predictor of conduct problems in boys whereas fearfulness was a stronger predictor in girls. Conduct problems also were robustly predicted by low levels of early mother-report cognitive stimulation when infant temperament was controlled. Interviewer-rated maternal responsiveness was a robust predictor of conduct problems, but only among infants low in fearfulness. Spanking during infancy predicted slightly more severe conduct problems, but the prediction was moderated by infant fussiness and positive affect. Thus, individual differences in risk for mother-rated conduct problems across childhood are already partly evident in maternal ratings of temperament during the first year of life and are predicted by early parenting and parenting-by-temperament interactions. [ABSTRACT FROM AUTHOR]

Recent studies suggest that most of what parents know about their adolescent offspring's whereabouts and companions is the result of youth disclosure, rather than information gained through active parental monitoring. This raises the possibility that parental knowledge is spuriously correlated with youth delinquency solely because the most delinquent youth disclose the least information to parents (because they have the most to hide). We tested this spurious association hypothesis using prospective data on offspring of a nationally representative sample of US women, controlling demographic and contextual covariates. In separate analyses, greater parental knowledge of their offspring's peer associations at both 12-13 years and at 14-15 years was associated with lower odds of being in the top 1 standard deviation of youth-reported delinquency at 16-17 years, controlling for delinquency at the earlier ages. The extent to which parents set limits on activities with peers at 14-15 years did not mediate or moderate the association between parental knowledge and delinquency, but it did independently predict future delinquency among adolescents living in high-risk neighborhoods. This suggests that the association between parental knowledge and future delinquency is not solely spurious; rather parental knowledge and limit setting are both meaningful predictors of future delinquency. [ABSTRACT FROM AUTHOR]

To contribute to our understanding of cohort differences and the Flynn effect in the cognitive decline among older Americans, this study aims to compare rates of cognitive decline between two birth cohorts within a study of older Americans and to examine the importance of medical and demographic confounders. Analyses used data from the National Health and Aging Trends Study (2011-2019), which recruited older Americans in 2011 and again in 2015 who were then followed for 5 years. We employed mixed-effect models to examine the linear and quadratic main and interaction effects of year of birth while adjusting for covariates such as annual round, sex/gender, education, race/ethnicity, heart disease, hypertension, diabetes, test unfamiliarity, and survey design. We analyzed data from 11,167 participants: 7,325 from 2011 to 2015 and 3,842 from 2015 to 2019. The cohort recruited in 2015 was born, on average, 5.33 years later than that recruited in 2011 and had higher functioning than the one recruited in 2011 across all observed cognitive domains that persisted after adjusting for covariates. In multivariable-adjusted analyses, a 1-year increase in year of birth was associated with increased episodic memory (β = 0.045, SE = 0.001, p < .001), orientation (β = 0.034, SE = 0.001, p < .001), and executive function (β = 0.036, SE = 0.001, p < .001). Participants born 1 year later had slower rates of decline in episodic memory (β = 0.004, SE = 0.000, p < .001), orientation (β = 0.003, SE = 0.000, p < .001), and executive function (β = 0.001, SE = 0.000, p = .002). Additionally, sex/gender modified this relationship for episodic memory (-0.007, SE = 0.002, p < .001), orientation (-0.005, SE = 0.002, p = .008), and executive function (-0.008, SE = 0.002, p < .001). These results demonstrate the persistence of the Flynn effect in old age across cognitive domains and identified a deceleration in the rate of cognitive decline across cognitive domains. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

In light of the "replication crisis," some advocate for stricter standards and greater transparency in research methods. These efforts push toward a data analysis approach called "confirmatory data analysis" (CDA; see Wagenmakers et al., 2012). However, some (e.g., Baumeister, 2016; Goldin-Meadow, 2016) suggest that emphasizing CDA may restrict creativity and discovery. These scholars argued (sometimes inadvertently) for greater freedom to pursue "exploratory data analysis" (EDA; see Tukey, 1977). Ironically and unfortunately, many who push against stricter CDA standards do not realize EDA exists, or misunderstand the philosophy and proper tools for exploration. In this article, the meaning, tools, philosophy, and ethics associated with EDA, CDA, and a relatively unknown but important approach called "rough CDA" are clarified. Important distinctions are developed between EDA/rough CDA/CDA and other (some problematic) analysis activities including p-hacking, HARKing, and data mining, which are situated in a (graphical) framework that clarifies relationships and ethical boundaries with each. In short, the proper data analytic approach depends on (a) intentions and (b) transparency. Most psychological research is not at a maturity level to justify CDA; researchers have historically used tools mismatched to their research agenda. In the conclusion, recommendations are presented about how these typologies can be integrated into graduate training programs and how a cumulative research program can help psychology move beyond the replication crisis. (PsycInfo Database Record (c) 2022 APA, all rights reserved).

This study evaluated changes over time in the quality of children's home environment, using the Home Observation Measurement of the Environment (HOME). Longitudinal increases in HOME scores were predicted by both theory and past empirical results. Analysis of the National Longitudinal Survey of Youth Children data (N = 5715, aged 0-14) suggested that HOME scores have been increasing, and that the increase is a family-level phenomenon. The data were a sample of children born to mothers who were approximately representative of the United States in 1979. An increase in HOME scores occurred primarily for the three age categories younger than ten. Effect sizes were of approximately the same magnitude as the Flynn effect for intelligence. These results have implications for policy and future research regarding the home environment., Competing Interests: Conflict of interestThe authors declare no competing interests., (© The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2021.)

Longitudinal studies have documented improvements in parents' life satisfaction due to childbearing, followed by postpartum adaptation back to baseline. However, the details underlying this process remain largely unexplored. Based on past literature, set-point theory, and results from an exploratory sample, we investigated empirically how first childbirth affected satisfaction with specific domains of life. In a preregistered study, we compared parents with matched childless respondents in their trajectories of life satisfaction, and also satisfaction with family life, health, sleep, work, housework, leisure, dwelling, household income, and personal income. First-time parents and childless respondents were matched in a procedure combining exact and propensity score matching. Using the population-representative German SOEP data ( N = 3,370), longitudinal multilevel models revealed heterogeneous effects of childbirth on different domains of satisfaction: Both mothers' and fathers' satisfaction with family life increased temporarily in a similar fashion to life satisfaction before going back to baseline within five years after childbirth. However, only mothers experienced drastic losses to satisfaction with sleep and satisfaction with personal income. For the remaining domains, parents' satisfaction largely resembled that of the matched childless respondents. These divergent domain trajectories underscore the need for multivariate analyses in life satisfaction research. (PsycInfo Database Record (c) 2021 APA, all rights reserved).

In 1918, Fisher suggested that his research team had consistently found inflated cousin correlations. He also commented that because a cousin sample with minimal selection bias was not available the cause of the inflation could not be addressed, leaving this inflation as a challenge still to be solved. In the National Longitudinal Survey of Youth (the NLSY79, the NLSY97, and the NLSY-Children/Young Adult datasets), there are thousands of available cousin pairs. Those in the NLSYC/YA are obtained approximately without selection. In this paper, we address Fisher's challenge using these data. Further, we also evaluate the possibility of fitting ACE models using only cousin pairs, including full cousins, half-cousins, and quarter-cousins. To have any chance at success in such a restricted kinship domain requires an available and highly-reliable phenotype; we use adult height in our analysis. Results provide a possible answer to Fisher's challenge, and demonstrate the potential for using cousin pairs in a stand-alone analysis (as well as in combination with other biometrical designs).

The National Longitudinal Survey of Youth datasets (NLSY79; NLSY-Children/Young Adults; NLSY97) have extensive family pedigree information contained within them. These data sources are based on probability sampling, a longitudinal design, and a cross-generational and within-family data structure, with hundreds of phenotypes relevant to behavior genetic (BG) researchers, as well as to other developmental and family researchers. These datasets provide a unique and powerful source of information for BG researchers. But much of the information required for biometrical modeling has been hidden, and has required substantial programming effort to uncover-until recently. Our research team has spent over 20 years developing kinship links to genetically inform biometrical modeling. In the most recent release of kinship links from two of the NLSY datasets, the direct kinship indicators included in the 2006 surveys allowed successful and unambiguous linking of over 94 % of the potential pairs. In this paper, we provide details for research teams interested in using the NLSY data portfolio to conduct BG (and other family-oriented) research.

A powerful longitudinal data source, the National Longitudinal Survey of Youth Children data, allows measurement of behavior problems (BP) within a developmental perspective linking them to menarcheal timing (MT). In a preliminary analysis, we evaluate the bivariate relationships between BP measured at different developmental periods and the timing of menarche. Correlations were not consistent with any correlational/causal relationship between BP and MT. In the major part of our study, MT was used to moderate the developmental trajectory of BP, within a genetically-informed design. Girls reaching menarche early had behavior problem variance accounted for by the shared environment; those reaching menarche with average/late timing had behavior problem differences accounted for by genetic variance. Our findings match previous empirical results in important ways, and also extend those results. A theoretical interpretation is offered in relation to a theory linking genetic/shared environmental variance to flexibility and choices available within the family in relation to BP.

Many data structures, particularly time series data, are naturally seasonal, cyclical, or otherwise circular. Past graphical methods for time series have focused on linear plots. In this article, we move graphical analysis onto the circle. We focus on 2 particular methods, one old and one new. Rose diagrams are circular histograms and can be produced in several different forms using the RRose software system. In addition, we propose, develop, illustrate, and provide software support for a new circular graphical method, called Wrap-Around Time Series Plots (WATS Plots), which is a graphical method useful to support time series analyses in general but in particular in relation to interrupted time series designs. We illustrate the use of WATS Plots with an interrupted time series design evaluating the effect of the Oklahoma City bombing on birthrates in Oklahoma County during the 10 years surrounding the bombing of the Murrah Building in Oklahoma City. We compare WATS Plots with linear time series representations and overlay them with smoothing and error bands. Each method is shown to have advantages in relation to the other; in our example, the WATS Plots more clearly show the existence and effect size of the fertility differential.

Although the Flynn Effect has been studied widely across cultural, geographic, and intellectual domains, and many explanatory theories have been proposed, little past research attention has been paid to subgroup differences. Rodgers and Wänström (2007) identified an aggregate-level Flynn Effect (FE) at each age between 5 and 13 in the Children of the National Longitudinal Survey of Youth (NLSYC) PIAT-Math data. FE patterns were not obtained for Reading Recognition, Reading Comprehension, or Digit Span, consistent with past FE research suggesting a closer relationship to fluid intelligence measures of problem solving and analytic reasoning than to crystallized measures of verbal comprehension and memory. These prior findings suggest that the NLSYC data can be used as a natural laboratory to study more subtle FE patterns within various demographic subgroups. We test for subgroup Flynn Effect differences by gender, race/ethnicity, maternal education, household income, and urbanization. No subgroups differences emerged for three demographic categories. However, children with more educated (especially college educated) mothers and/or children born into higher income households had an accelerated Flynn effect in their PIAT-M scores compared to cohort peers with lower educated mothers or lower income households. We interpret both the positive and the null findings in relation to previous theoretical explanations.

We examine how the motivational sequence that leads to childbearing predicts fertility outcomes across reproductive careers. Using a motivational traits-desires-intentions theoretical framework, we test a structural equation model using prospective male and female data from the National Longitudinal Survey of Youth. Specifically, we take motivational data collected during the 1979-1982 period, when the youths were in their teens and early twenties, to predict the timing of the next child born after 1982 and the total number of children born by 2002. Separate models were estimated for males and females but ivith equality constraints imposed unless relaxing these constraints improved the overall model fit. The results indicate substantial explanatory power of fertility motivations for both short-term and long-term fertility outcomes. They also reveal the effects of both gender role attitude and educational intentions on these outcomes. Although some gender differences in model pathways occurred, the primary hypothesized pathways were essentially the same across the genders. Two validity substudies support the soundness of the results. A third sub-study comparing the male and female models across the sample split on the basis of previous childbearing revealed a number of pattern differences within the four gender-by-previous childbearing groups. Several of the more robust of these pattern differences offer interesting insights and support the validity and usefulness of our theoretical framework.

A quiet methodological revolution, a modeling revolution, has occurred over the past several decades, almost without discussion. In contrast, the 20th century ended with contentious argument over the utility of null hypothesis significance testing (NHST). The NHST controversy may have been at least partially irrelevant, because in certain ways the modeling revolution obviated the NHST argument. I begin with a history of NHST and modeling and their relation to one another. Next, I define and illustrate principles involved in developing and evaluating mathematical models. Following, I discuss the difference between using statistical procedures within a rule-based framework and building mathematical models from a scientific epistemology. Only the former is treated carefully in most psychology graduate training. The pedagogical implications of this imbalance and the revised pedagogy required to account for the modeling revolution are described. To conclude, I discuss how attention to modeling implies shifting statistical practice in certain progressive ways. The epistemological basis of statistics has moved away from being a set of procedures, applied mechanistically, and moved toward building and evaluating statistical and scientific models., (Copyrigiht 2009 APA, all rights reserved.)

Using National Longitudinal Survey of Youth (NLSY) fertility variables, we introduce and illustrate a new genetically-informative design. First, we develop a kinship linking algorithm, using the NLSY79 and the NLSY-Children data to link mothers to daughters and aunts to nieces. Then we construct mother-daughter correlations to compare to aunt-niece correlations, an MDAN design, within the context of the quantitative genetic model. The results of our empirical illustration, which uses DF Analysis and generalized estimation equations (GEE) to estimate biometrical parameters from NLSY79 sister-sister pairs and their children in the NLSY-Children dataset, provide both face validity and concurrent validity in support of the efficacy of the design. We describe extensions of the MDAN design. Compared to the typical within-generational design used in most behavior genetic research, the cross-generational feature of this design has certain advantages and interesting features. In particular, we note that the equal environment assumption of the traditional biometrical model shifts in the context of a cross-generational design. These shifts raise questions and provide motivation for future research using the MDAN and other cross-generational designs.

The authors study education and cognitive ability as predictors of female age at first birth (AFB), using monozygotic and dizygotic female twin pairs from the Middle-Aged Danish Twin survey. Using mediated regression, they replicate findings linking education (and not cognitive ability) to AFB. But in a behavior genetic model, both relationships are absorbed within a latent variable measuring the shared family environment. Two interpretations are relevant. First, variance in AFB emerges from differences between families, not differences between sisters within the same family. Second, even in a natural laboratory sensitive to genetic variance in female fertility -- during demographic transition -- the variance in AFB was non-genetic, located instead within the shared environment.

This article proposes 2 new approaches to test a nonzero population correlation (rho): the hypothesis-imposed univariate sampling bootstrap (HI) and the observed-imposed univariate sampling bootstrap (OI). The authors simulated correlated populations with various combinations of normal and skewed variates. With alpha set=.05, N> or =10, and rho< or =0.4, empirical Type I error rates of the parametric r and the conventional bivariate sampling bootstrap reached .168 and .081, respectively, whereas the largest error rates of the HI and the OI were .079 and .062. On the basis of these results, the authors suggest that the OI is preferable in alpha control to parametric approaches if the researcher believes the population is nonnormal and wishes to test for nonzero rhos of moderate size., (Copyright (c) 2008 APA.)

Context: Existing research on the neurobehavioral consequences of prenatal alcohol exposure (PAE) has not adequately accounted for genetic and environmental confounds., Objective: To examine the association between PAE and offspring externalizing problems in a large representative sample of families in the United States using measured covariates and a quasi-experimental design to account for unmeasured genetic and environmental confounds., Design: This study combines information from the National Longitudinal Survey of Youth and the Children of the National Longitudinal Survey of Youth. The analyses statistically controlled for measured characteristics of the mothers and families and exposure to other prenatal psychoactive substances. In the primary analyses, siblings differentially exposed to prenatal alcohol were compared., Setting and Participants: Women were recruited from the community using a stratified and clustered probability sample and were followed longitudinally. The sample included 8621 offspring of 4912 mothers., Main Outcome Measures: Maternal report of conduct problems (CPs) and attention/impulsivity problems (AIPs) during childhood (ages 4-11 years) using standardized assessments related to psychiatric diagnoses., Results: There was an association between PAE and offspring CPs that was independent of confounded genetic and fixed environmental effects and the measured covariates. The CPs in children of mothers who drank daily during pregnancy were 0.35 SD greater than those in children whose mothers never drank during pregnancy. Although AIPs were associated with PAE when comparing unrelated offspring, children whose mothers drank more frequently during pregnancy did not have more AIPs than siblings who were less exposed to alcohol in utero. Additional subsample analyses suggested that maternal polysubstance use during pregnancy may account for the associations between PAE and AIPs., Conclusion: These results are consistent with PAE exerting an environmentally mediated causal effect on childhood CPs, but the relation between PAE and AIPs is more likely to be caused by other factors correlated with maternal drinking during pregnancy.

We address points raised by Zajonc and Sulloway, who reject findings showing that birth order has no effect on intelligence. Many objections to findings of null birth-order results seem to stem from a misunderstanding of the difference between study designs where birth order is confounded with true causal influences on intelligence across families and designs that control for some of these influences. We discuss some of the consequences of not appreciating the nature of this difference. When between-family confounds are controlled using appropriate study designs and techniques such as multilevel modeling, birth order is shown not to influence intelligence. We conclude with an empirical investigation of the replicability and generalizability of this approach.

Substantial evidence now exists that variables measuring or correlated with human fertility outcomes have a heritable component. In this study, we define a series of age-sequenced fertility variables, and fit multivariate models to account for underlying shared genetic and environmental sources of variance. We make predictions based on a theory developed by Udry [(1996) Biosocial models of low-fertility societies. In: Casterline, JB, Lee RD, Foote KA (eds) Fertility in the United States: new patterns, new theories. The Population Council, New York] suggesting that biological/genetic motivations can be more easily realized and measured in settings in which fertility choices are available. Udry's theory, along with principles from molecular genetics and certain tenets of life history theory, allow us to make specific predictions about biometrical patterns across age. Consistent with predictions, our results suggest that there are different sources of genetic influence on fertility variance at early compared to later ages, but that there is only one source of shared environmental influence that occurs at early ages. These patterns are suggestive of the types of gene-gene and gene-environment interactions for which we must account to better understand individual differences in fertility outcomes.

Accurate descriptions of sex differences in the development of childhood conduct problems and adolescent delinquency will inform theories of their causes in fundamentally important ways. Using data on 4,572 offspring of a national sample of women, we tested descriptive hypotheses regarding sex differences. As predicted, the magnitude of sex differences varied with age, suggesting that multiple processes differentially influence levels of these behaviors in females and males across development. During childhood, boys scored lower on measures of cognitive ability and exhibited lower sociability and compliance and greater hyperactivity, oppositional behavior, and conduct problems. Most of these variables were associated with childhood conduct problems and adolescent delinquency equally in females and males, but maternal delinquency and early childhood sociability were correlated more strongly with childhood conduct problems in males and childhood compliance predicted adolescent delinquency more strongly in females. Both sexes exhibited both childhood-onset and adolescent-onset trajectories of delinquency. Although more males followed a childhood-onset trajectory, there were few sex differences in the early childhood risk correlates of either delinquency trajectory.

Many studies show relationships between birth order and intelligence but use cross-sectional designs or manifest other threats to internal validity. Multilevel analyses with a control variable show that when these threats are removed, two major results emerge: (a) birth order has no significant influence on children's intelligence and (b) earlier reported birth order effects on intelligence are attributable to factors that vary between, not within, families. Analyses on 7- to 8 - and 13- to 14-year-old children from the National Longitudinal Survey of Youth support these conclusions. When hierarchical data structures, age variance of children, and within-family versus between-family variance sources are taken into account, previous research is seen in a new light.

Political and sociocultural events (e.g., Brown v. Board of Education in 1954 and the German reunification in 1989) and natural disasters (e.g., Hurricane Hugo in 1989) can affect fertility. In our research, we addressed the question of whether the Oklahoma City bombing in April 1995, a man-made disaster, influenced fertility patterns in Oklahoma. We defined three theoretical orientations--replacement theory, community influence theory, and terror management theory--that motivate a general expectation of birth increases, with different predictions emerging from time and geographic considerations. We used two different empirical methodologies. First, we fitted dummy-variable regression models to monthly birth data from 1990 to 1999 in metropolitan counties. We used birth counts to frame the problem and general fertility rates to address the problem formally. These analyses were organized within two design structures: a control-group interrupted time-series design and a difference-in-differences design. In these analyses, Oklahoma County showed an interpretable, consistent, and significant increase in births. Second, we used graphical smoothing models to display these effects visually. In combination, these methods provide compelling support for a fertility response to the Oklahoma City bombing. Certain parts of each theory helped us organize and understand the pattern of results.

Previous research has demonstrated that U.S. no-fault divorce laws implemented between 1953 and 1987 resulted in more divorces in some states than would have occurred otherwise. In other states, divorce patterns appeared to follow prevailing trends even after implementation of no-fault divorce legislation. A more distal question is whether implementation of no-fault divorce laws had an effect on birth rates. We analyzed state-level birth data from all 50 states to assess the birth response to the enactment of no-fault divorce law in each state. Results suggested that birth rates decreased significantly two to four years following the enactment of no-fault divorce law for the group of 34 states whose divorce rates responded to no-fault divorce legislation. As predicted, among the 16 states whose divorce rates did not respond to no-fault divorce legislation, the enactment of no-fault divorce law had a small and nonsignificant positive influence on birth rates. Generally, the group of 34 states had lower post no-fault birth rates than the group of 16 states.

Behaviors related to fertility constitute primary candidates for investigating the relevance of evolutionary influences and biological dispositions on contemporary human behaviors. Using female Danish twin cohorts born 1870-1968, we document important transformations in the relative contributions of "nurture" and "nature" to within-cohort variations in early and complete fertility, and we point toward a systematic relation between the socioeconomic context of cohorts and the relevance of genetic and shared environmental factors. This transformation is most striking for early fertility where genetic factors strengthen over time and are consistent with up to 50 percent of the variation in early fertility in most recent cohorts. Understanding this emerging relevance of genetic factors is of central importance because early fertility constitutes an important determinant of complete fertility levels in low-fertility societies, and because teenage motherhood and early childbearing are often associated with negative life-cycle consequences. Moreover, our results emphasize the need for socially and contextually informed analyses of nature and nurture that allow both factors to influence human reproductive behavior over time.

S. Roberts and H. Pashler (2000) argued against using goodness of fit as evidence to support theories. The authors agree with their suggestions for how to go beyond good fits but disagree with their starting point. In this comment, the authors argue that good fits are part and parcel of theory development, that they are part and parcel of the processes suggested by S. Roberts and H. Pashler, and that they must be the starting point (though far from the ending point) in theoretical development. The authors discuss historical examples of scientific theory development, recent examples of psychological theory development, and development of a particular theory (social contagion theory; J. L. Rodgers & D. C. Rowe, 1993) that S. Roberts and H. Pashler criticized.