Genetic and Environmental Correlates of Physical and Cognitive Development in Twins: A Prospective Study of Recovery from Bio-Environmental Adversity

Relative to singletons, twins are at an elevated risk to be born prematurely and at low birth weight. Consequently, twins are small relative to population norms on anthropomorphic measurements in infancy and score about a standard deviation below the population mean on early cognitive assessments. However, by middle childhood twins are average physically and cognitively. Although twins make gains toward the population mean physically and cognitively over the first years of life, the recovery process is not well understood. The goal of this dissertation is to explore the process of physical and cognitive recovery using prospective data from a community sample of twins followed from infancy to adolescence. To that end, I fit a series of growth models to age-standardized measurements of height, weight, head circumference, and cognitive ability to identify typical trajectories of physical and cognitive recovery in twins. Leveraging the genetically informative portion of the data, I conducted biometric analyses to determine the proportion of the variance of recovery associated with genetic and environmental factors. I then tested associations between aspects of the environment (e.g., gestational age, family socioeconomic status) and patterns of recovery. Finally, I explored the co-development of physical size and cognitive ability using models of parallel development and dynamic development.

Recovery of height, weight, and head circumference was most rapid in the first year of life whereas cognitive recovery accelerated in toddlerhood. Shared environmental factors explained the majority of the variance in early physical size and cognitive ability. The rate, shape, and magnitude of physical and cognitive recovery was related to a combination of genetic and environmental factors. Associations between physical and cognitive development were moderate to strong in infancy, but nonsignificant by adolescence. There was some evidence of a dynamic relationship between physical recovery and cognitive development in infancy with larger height and head circumference measurements leading to increases in the rate of cognitive growth. Although the present study focuses on twins, twins can serve as a model for singleton development. Findings from this dissertation inform our understanding of typical patterns of physical and cognitive recovery following exposure to early bioenvironmental adversity, which can be used to identify children displaying atypical development.