The cross-generational mother-daughter-aunt-niece design: establishing validity of the MDAN design with NLSY fertility variables

The Influence of a Sister's Breastfeeding Experience on a Mother's Breastfeeding Behavior: Is There an Intragenerational Effect?

Objective: Intergenerational transmission of breastfeeding attitudes and behaviors from mother to daughter are well known, but there is limited research on intragenerational transmission of breastfeeding attitudes or behaviors within families. This study aimed to understand how initiation and duration of breastfeeding are influenced by past breastfeeding experiences of sisters among women in a longitudinal population-based cohort. Methods: Data were obtained on women enrolled in the 1979 National Longitudinal Survey of Youth (NLSY) who had at least one child enrolled in a substudy of cohort members' children. For each mother in the study, we determined whether any of her sisters also enrolled in the main NLSY study had previously breastfed one of their children. Results: Mothers whose sister(s) had prior breastfeeding experience were more likely to breastfeed their first-born child on unadjusted analysis (70% versus 45%, chi-square p < 0.001) and had a longer median of breastfeeding duration (median 14.5 versus 12 weeks, rank-sum p = 0.039). However, on a multivariable analysis accounting for potential confounding by maternal characteristics, infant characteristics, and differences among households, sisters' breastfeeding experience was no longer independently associated with the likelihood of breastfeeding initiation (odds ratio: 1.16; confidence interval [95% CI]: 0.73-1.85; p = 0.520) or the hazard of breastfeeding discontinuation (hazard ratio: 0.96; 95% CI: 0.82-1.12; p = 0.598). Conclusion: After adjusting for socioeconomic characteristics, intragenerational transmission of breastfeeding behavior was negligible among mothers raised in the same household. Other forms of intragenerational transmission (e.g., influence of extended family members) may be more salient influences on women's decision to breastfeed.

The Analytic Identification of Variance Component Models Common to Behavior Genetics

Many behavior genetics models follow the same general structure. We describe this general structure and analytically derive simple criteria for its identification. In particular, we find that variance components can be uniquely estimated whenever the relatedness matrices that define the components are linearly independent (i.e., not confounded). Thus, we emphasize determining which variance components can be identified given a set of genetic and environmental relationships, rather than the estimation procedures. We validate the identification criteria with several well-known models, and further apply them to several less common models. The first model distinguishes child-rearing environment from extended family environment. The second model adds a gene-by-common-environment interaction term in sets of twins reared apart and together. The third model separates measured-genomic relatedness from the scanner site variation in a hypothetical functional magnetic resonance imaging study. The computationally easy analytic identification criteria allow researchers to quickly address model identification issues and define novel variance components, facilitating the development of new research questions.

Responding to a 100-Year-Old Challenge from Fisher: A Biometrical Analysis of Adult Height in the NLSY Data Using Only Cousin Pairs

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 NLSY Kinship Links: Using the NLSY79 and NLSY-Children Data to Conduct Genetically-Informed and Family-Oriented Research

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.

Clarifying the associations between age at menarche and adolescent emotional and behavioral problems

Better understanding risk factors for the development of adolescent emotional and behavioral problems can help with intervention and prevention efforts. Previous studies have found that an early menarcheal age predicts several adolescent problems, including depressive symptoms, delinquency, and early age at first intercourse. Few studies, nevertheless, have explicitly tested (a) whether the associations with menarcheal age vary across racial/ethnic groups or (b) whether the sources of the associations are within-families (i.e., consistent with a direct, causal link) or only between-families (i.e., due to selection or confounding factors). The current study analyzed data from a nationally representative US Sample of females (N = 5,637). We examined whether race/ethnicity moderated the associations between early menarche and several adolescent problems by using multiple-group analyses and we examined the degree to which genetic and environmental factors shared by family members account for the associations by comparing sisters and cousins with differing menarcheal ages. Menarcheal age predicted subsequent depressive symptoms, delinquency, and early age at first intercourse in the population. The magnitudes of the associations were similar across all racial/ethnic groups for all outcomes. The within-family associations (i.e., when comparing siblings and cousins with different menarcheal age) were large and statistically significant when predicting early intercourse, but not the other outcomes. The findings suggest that selection or confounding factors account for the associations between menarcheal age and subsequent depressive symptoms and delinquency, whereas the independent association between menarcheal age and early age at first intercourse is consistent with a direct, causal effect.

Behavior problems and timing of menarche: a developmental longitudinal biometrical analysis using the NLSY-Children data

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.

Adolescent sexual activity and the development of delinquent behavior: the role of relationship context

Despite the well-established association between adolescent sexual activity and delinquent behavior, little research has examined the potential importance of relationship contexts in moderating this association. The current study used longitudinal, behavioral genetic data on 519 same-sex twin pairs (48.6% female) divided into two age cohorts (13-15 and 16-18 years olds) drawn from the National Longitudinal Study of Adolescent Health. Analyses tested whether adolescent sexual activity that occurred in romantic versus non-romantic relationships was associated with delinquency from adolescence to early adulthood, after controlling for genetic influences. Results indicated that, for both younger and older adolescents, common underlying genes influence both sexual behavior and delinquency. After controlling for these genetic influences, there was no within-twin pair association between sexual activity and delinquency in younger adolescents. In older adolescents, sexual activity that occurred in romantic relationships predicted lower levels of delinquency, both cross-sectionally and longitudinally, whereas sexual activity in non-romantic relationships predicted higher levels of delinquency. These results are consistent with emerging research that suggests that the psychological correlates of adolescent sexual activity may be moderated by the social context in which this activity occurs.

Biodemographic modeling of the links between fertility motivation and fertility outcomes in the NLSY79

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.

Education and cognitive ability as direct, mediating, or spurious influences on female age at first birth: behavior genetic models fit to Danish twin data

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.