UC Davis

Research on developmental plasticity has highlighted individual differences in susceptibility to both positive and negative environmental exposures and developmental experiences; however, it has been unclear (1) whether there are two types of people in terms of developmental plasticity (i.e., plastic, fixed) and (2) whether such plasticity is trait-like and thus domain general (i.e., individuals may be consistently more or less susceptible to many different experiences and exposures and with respect to many different aspects of development) or domain specific (i.e., individuals may be susceptible to certain environmental influences on certain aspects of development). The current dissertation empirically addresses these two questions in two studies, both now published online (Zhang, Widaman, & Belsky, 2021; Zhang, Schlomer, Ellis, & Belsky, 2021), which draw data from the NICHD Study of Early Child Care and Youth Development (N = 1364), while applying a novel influence-statistics’ method to quantify individual differences in environmental susceptibility.

Paper 1 focused on children’s susceptibility to potential family and childcare influences. In particular, this work mainly aimed to investigate the following issues: (1) whether susceptibility to multiple environmental effects was bimodally or normally distributed; (2a) whether children more and less susceptible to family effects proved similarly susceptible to childcare effects; (2b) whether children’s cognitive-linguistic and social-behavioral development proved similarly affected by different contextual effects; and, finally, (3) whether infant temperamental difficulty and genetic make-up helped to account for variation in developmental plasticity, that is, susceptibility to environmental effects.

These issues were addressed in Study 1 by relying on effects of 5 family predictors (e.g., sensitive parenting) and 3 childcare exposures (e.g., childcare quality) measured between 3 and 54 months of age on 5 developmental outcomes assessed at 4.5 years (e.g., problem behavior). Results indicated that susceptibility across the investigated associations was normally distributed, suggesting the dual-person view of developmental plasticity may be untenable. Evidence also provided support for both domain generality and specificity of developmental plasticity, with effect sizes proving smaller in the former case. Findings also showed that children with more difficult temperaments during infancy or who scored higher on a polygenic-plasticity score (5-HTTLPR, DRD4, BDNF) proved more susceptible to some of the environmental effects investigated.

Paper 2 focused on mother and adolescent’s susceptibility to effects of environmental harshness and unpredictability, also within the context of a longitudinal, observational research design (i.e., non-genetically sensitive). In particular, it investigated the extent to which harshness and unpredictability, measured from 1 to 54 months of children, predicted mother’s psychological well-being and parenting, as well as their adolescent’s life-history strategy (i.e., fast vs. slow), as reflected in the number of sexual partners by age 15 years. Results indicated that mothers and adolescents who proved more and less susceptible to harshness also proved more and less susceptible to unpredictability, but such seemingly domain-generality was not evident when considering mother’s susceptibility with respect to different outcomes.

Overall, this dissertation provided evidence (1) that susceptibility to (at least some) putative environmental influences is normally rather than bimodally distributed, suggesting a gradient in developmental plasticity, (2) while documenting both domain generality and specificity of developmental plasticity. Additionally, this dissertation showed how influence-statistics could be used to productively quantify individual differences in susceptibility to environmental influences, thereby opening up new avenues of inquiry with respect to the enduring issue of developmental plasticity.