Twin Differentiation of Cognitive Ability Through Phenotype to Environment Transmission: The Louisville Twin Study

Abstract

The Louisville Twin Study is one of the most intensive twin studies of cognitive ability. The repeated measurements of the twins are ideal for testing developmental twin models that allow for the accumulation of gene-environment correlation via a phenotype to environment (P=>E) transmission process to explain twins’ divergence in mean ability level over time. Using full-scale IQ scores from 566 pairs of twins (MZ=278; DZ=288), we tested whether a P=>E transmission model provided better representation of actual developmental processes than a genetic simplex model. We also addressed whether the induced gene-environment correlation alters the meaning of the latent nonshared environmental factors with a simple numerical method for interpreting nonshared environmental factors in the context of P=>E transmission. The results suggest that a P=>E model provided better fit to twins’ FSIQ data than a genetic simplex model and the meaning of the nonshared environment was preserved in the context of P=>E.

How nonshared environmental factors come to correlate with heredity

“…Since b PE parameters necessarily change the meaning of the nonshared environment at all days but the first day via indirect correlations between genetic and environmental components (Beam et al, 2015), we empirically test the longitudinal structure of the nonshared environment by statistically comparing model implied nonshared environmental variances and covariances in the DZ group against the MZ group.…”

“…As a result, the more affectively labile sibling may contribute to a social environment characterized by more conflict, potentially leading to greater lability over time. Although P→E models have not yet been applied to affect, we have demonstrated that small within-family phenotypic differences put genetically related siblings onto different cognitive (Beam et al, 2015) and personality (Beam et al, submitted) trajectories.…”

“…Several efforts have been made to understand within-family diversification, or "sibling drift," by incorporating gene-environment correlation in longitudinal twin models (e.g., Beam & Turkheimer, 2013;de Kort et al, 2012;Dolan et al, 2014;Neale & Cardon, 1992). Reciprocal effects models (REMs), in particular, have guided our thinking about change processes (Beam, Turkheimer, Dickens, & Davis, 2015;Beam & Turkheimer, 2013;Dickens, Turkheimer, & Beam, 2011). REMs are not novel (Bell, 1968;Scarr, 1992;Winship & Korenman, 1999) and summarize how small phenotypic advantages (e.g., calm demeanor) translate into superior abilities (e.g., affective stability).…”

“…Accounting for gene-environment correlation in longitudinal twin models can explain how environmental processes lead to increases in heritability, as well as misspecification of the longitudinal stability of nonshared environmental components. One consequence of model estimated gene-environment correlation, however, is that the interpretation of nonshared environmental components changes (Beam et al, 2015;de Kort et al, 2012). In P→E models, each nonshared environmental component in longitudinal twin models consists of two components: a part that correlates with genotype (i.e., it differs due to genotypic differences) and a part that is uncorrelated with genotype (i.e., occasion-specific residuals).…”

Cultural Evolution of Genetic Heritability

“…Several researchers (e.g., Bates, Lewis, and Weiss 2013;Beam et al 2015) have suggested that gene-environment correlation via phenotype-to-environment transmission, otherwise referred to as 'reciprocal causation' (Dickens and Flynn 2001;Bronfenbrenner and Ceci 1994;Scarr 1992), is the most likely explanation. By this explanation, those with genes well suited to a task can better nurture their skills in a wealthier environment than in a poorer environment.…”