8 Theoretical perspectives on the interplay of nature and nurture in personality development Christian Kandler 1,2 and Alexandra Zapko-Willmes 1,2 1 Bielefeld University, Bielefeld, Germany, 2 Medical School Berlin, Berlin, Germany Genetic and environmental variance in personality characteristics The claim that “all human behavioral traits are heritable” (Turkheimer, 2000, p. 160) is known as the first law of behavior genetics and means that genetic differences matter regarding individual differences in all human characteristics. In their meta-analysis of the genetic and environmental influences on individual differences in 17,804 human traits based on 50 years of twin studies, Polderman et al. (2015) reported that the average heritability across all complex traits is 49%. Consequently, about one half of individual differences across all focused traits was attributable to genetic differences (see Kandler & Papendick, Chapter 29). On average, only about 17% of variance in human traits were due to environmental influences that act to increase the similarity of same-aged siblings (i.e., monozygotic and dizygotic twins) reared together. These findings have two important implications. First, the similarity of biologically related family members in complex human behavioral traits is primarily attributable to their genetic relatedness. This has been formulated as the second law of behavior genetics: “The effect of being raised in the same family is smaller than the effect of genes” (Turkheimer, 2000, p. 160). Second, a substantial residual portion of individual differences in complex human behavioral traits can be accounted for by factors that are not shared by family members and act to make them dissimilar. The latter has come to be known as the third law of behavior genetics: “A substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families” (Turkheimer, 2000, p. 160). Despite some variation in the size of heritability estimates, the average heritability of psychological features commonly termed “personality characteris- tics” also amounts to 50% (Bouchard, 2004; Johnson, Vernon, & Feiler, 2008). Additionally, both little evidence for significant environmental influences shared by family members and strong influences of individual environmental factors apply to personality traits. These findings do not exclusively stem from aggregates of self-rated personality-descriptive adjectives or statements, but also from behavioral Personality Development Across the Lifespan. DOI: http://dx.doi.org/10.1016/B978-0-12-804674-6.00008-9 © 2017 Elsevier Inc. All rights reserved.