What evidence is there to prefer the more complex explanation involving gene-environment dynamics over independent and additive effects of genes and the environment? As discussed in the introduction to this article, empirical examples of gene-environment dynamics are too common to ignore (see Dick, 2011; Johnson, 2007; Tabery, 2007). Gene-by-environment interactions “are ubiquitous in nature” and evolutionarily adaptive (Penke, Denissen, & Miller, 2007, p. 578). Phenotypic plasticity in response to environmental circumstances and resources allow organisms to adjust to environmental variation (Dall, Houston, & McNamara, 2004). This feature may explain the extremely high heritability of personality in very early childhood. During this period, the ultimate phenotype is very amenable to environmental inputs, but the phenotype responds differentially on the basis of genotype. As this extreme plasticity subsides, other etiological mechanisms may begin to emerge as impactful on personality development. Future research that documents gene-environment interplay, as well as, direct genetic and environmental effects will be important to more precisely answer the magnitude of the importance of the various processes.
