Rolf Degen summarizing... With increasing age, individual differences in personality traits are more and more determined by idiosyncratic life experiences, compared to genes and family upbringing

Kandler, C., Bratko, D., Butković, A., Hlupić, T. V., Tybur, J. M., Wesseldijk, L. W., de Vries, R. E., Jern, P., & Lewis, G. J. (2020). How genetic and environmental variance in personality traits shift across the life span: Evidence from a cross-national twin study. Journal of Personality and Social Psychology, Sep 2020. https://doi.org/10.1037/pspp0000366

Rolf Degen's take: https://twitter.com/DegenRolf/status/1309515043860549634

Abstract: Decades of research have shown that about half of individual differences in personality traits is heritable. Recent studies have reported that heritability is not fixed, but instead decreases across the life span. However, findings are inconsistent and it is yet unclear whether these trends are because of a waning importance of heritable tendencies, attributable to cumulative experiential influences with age, or because of nonlinear patterns suggesting Gene × Environment interplay. We combined four twin samples (N = 7,026) from Croatia, Finland, Germany, and the United Kingdom, and we examined age trends in genetic and environmental variance in the six HEXACO personality traits: Honesty-Humility, Emotionality, Extraversion, Agreeableness, Conscientiousness, and Openness. The cross-national sample ranges in age from 14 to 90 years, allowing analyses of linear and nonlinear age differences in genetic and environmental components of trait variance, after controlling for gender and national differences. The amount of genetic variance in Extraversion, Agreeableness, and Openness followed a reversed U-shaped pattern across age, showed a declining trend for Honesty-Humility and Conscientiousness, and was stable for Emotionality. For most traits, findings provided evidence for an increasing relative importance of life experiences contributing to personality differences across the life span. The findings are discussed against the background of Gene × Environment transactions and interactions.