Kathryn Paige Harden

Sleep duration and body mass index in twins: a gene-environment interaction.

STUDY OBJECTIVES To examine whether sleep duration modifies genetic and environmental influences on body mass index (BMI). DESIGN Genotype-environment interaction twin study. SETTING University of Washington Twin Registry. PATIENTS OR PARTICIPANTS A population-based sample of US twins (1,088 pairs, 604 monozygotic, 484 dizygotic; 66% female; mean age = 36.6 yr, standard deviation (SD) = 15.9 yr). INTERVENTIONS N/A. MEASUREMENTS AND RESULTS Participants self-reported information on height, weight, and sleep. Mean BMI was calculated as 25.3 kg/m² (SD = 5.4) and mean habitual sleep duration was 7.2 hr/night (SD = 1.2). Data were analyzed using biometric genetic interaction models. Overall the heritability of sleep duration was 34%. Longer sleep duration was associated with decreased BMI (P < 0.05). The heritability of BMI when sleep duration was < 7 hr (h² = 70%) was more than twice as large as the heritability of BMI when sleep duration was ≥ 9 hr (h² = 32%); this interaction was significant (P < 0.05). CONCLUSIONS Shorter sleep duration is associated with increased BMI and increased genetic influences on BMI, suggesting that shorter sleep duration increases expression of genetic risks for high body weight. At the same time, longer sleep duration may suppress genetic influences on body weight. Future research aiming to identify specific genotypes for BMI may benefit by considering the moderating role of sleep duration.

Pubertal Development and Peer Influence on Risky Decision Making

Adolescents engage in more risky behavior when they are with peers and show, on average, heightened susceptibility to peer influence relative to children and adults. However, individual differences in susceptibility to peer influence are not well understood. The current study examined whether the effect of peers on adolescents’ risky decision making was moderated by pubertal status. Participants (58 youth, ages 11-16, 50% male, 63.9% African American) completed a computerized measure of risky decision making, once alone and once in the presence of two peers. Pubertal status was assessed using self-report. Adolescents made riskier decisions in the presence of peers, and more advanced pubertal development predicted greater risky decision making, controlling for chronological age. The effect of peer presence on risky decision making was attenuated for adolescents with more advanced pubertal development. These findings suggest that the presence of peers may override biologically based individual differences in propensity for risk taking.

Why Don't Smart Teens Have Sex? A Behavioral Genetic Approach

Academic achievement and cognitive ability have been shown to predict later age at first sexual intercourse. Using a sample of 536 same-sex twin pairs who were followed longitudinally from adolescence to early adulthood, this study tested whether relations between intelligence, academic achievement, and age at first sex were due to unmeasured genetic and environmental differences between families. Twins who differed in their intelligence or their academic achievement did not differ in their age at first sex. Rather, the association between intelligence and age at first sex could be attributed entirely to unmeasured environmental differences between families, whereas the association between academic achievement and age at first sex could be attributed entirely to genetic factors.

Genetic influences on hormonal markers of chronic hypothalamic–pituitary–adrenal function in human hair

BACKGROUND Cortisol is the primary output of the hypothalamic-pituitary-adrenal (HPA) axis and is central to the biological stress response, with wide-ranging effects on psychiatric health. Despite well-studied biological pathways of glucocorticoid function, little attention has been paid to the role of genetic variation. Conventional salivary, urinary and serum measures are strongly influenced by diurnal variation and transient reactivity. Recently developed technology can be used to measure cortisol accumulation over several months in hair, thus indexing chronic HPA function. METHOD In a socio-economically diverse sample of 1070 twins/multiples (ages 7.80-19.47 years) from the Texas Twin Project, we estimated effects of sex, age and socio-economic status (SES) on hair concentrations of cortisol and its inactive metabolite, cortisone, along with their interactions with genetic and environmental factors. This is the first genetic study of hair neuroendocrine concentrations and the largest twin study of neuroendocrine concentrations in any tissue type. RESULTS Glucocorticoid concentrations increased with age for females, but not males. Genetic factors accounted for approximately half of the variation in cortisol and cortisone. Shared environmental effects dissipated over adolescence. Higher SES was related to shallower increases in cortisol with age. SES was unrelated to cortisone, and did not significantly moderate genetic effects on either cortisol or cortisone. CONCLUSIONS Genetic factors account for sizable proportions of glucocorticoid variation across the entire age range examined, whereas shared environmental influences are modest, and only apparent at earlier ages. Chronic glucocorticoid output appears to be more consistently related to biological sex, age and genotype than to experiential factors that cluster within nuclear families.

Gene × environment interactions in early externalizing behaviors : parental emotional support and socioeconomic context as moderators of genetic influences?

This study uses longitudinal population-based samples of young siblings to examine the effects of two hypothesized moderators of early externalizing behaviors: parental emotional support and family socioeconomic status. The first sample, a twin sample from the Early Childhood Longitudinal Study-Birth Cohort (ECLS-B), was composed of approximately 600 twin pairs measured on externalizing at ages 4 and 5. Results indicated stronger genetic influences on externalizing at lower levels of parental emotional support but higher levels of socioeconomic status; only the latter interaction remained significant when the two moderators were simultaneously modeled. These moderation effects were not replicated in our analyses of the National Longitudinal Survey of Youth-Child Supplement (CNLSY) data, which contained 1939 pairs of full and half siblings measured on externalizing at ages 4–5 and ages 6–7. Our results highlight the need for replication in quantitative behavior genetics research on externalizing behaviors. Potential causes for non-replication are discussed.