Individuals reared together are no more similar to one another in their personalities than if chosen at random in the population

Theoretical Concepts in the Genetics of Personality Development. Elliot M. Tucker-Drob & Daniel A. Briley. To appear in Handbook of Personality Development, by Dan P. McAdams, Rebecca L. Shiner, and Jennifer L. Tackett (Eds.). June 2017. http://labs.la.utexas.edu/tucker-drob/files/2015/02/Tucker-Drob-Briley-Genetics-of-Personality-Development-Chapter.pdf

Conventional work in the behavioral genetics of personality largely focused on single point estimates of heritability of personality. For instance, point estimates for the heritability of all of the Big Five personality traits have been reported to be approximately .40-.60 (for a review see Bouchard & McGue, 2003), with no consistent differences reported across different Big Five traits (Turkheimer, Pettersson, & Horn, 2014). Evidence for genetic influences on personality are derived from the observation that genetically more related individuals (e.g. identical twins) are more similar in their personality traits than genetically less related individuals (e.g. fraternal twins), even when holding shared rearing environment constant across relationship types. Also of note is that, after accounting for genetic relatedness, individuals reared together are no more similar to one another in their personalities than would be expected for individuals chosen at random out of the population. Nongenetic factors that differentiate individuals regardless of whether their rearing environment was shared with one another are termed the nonshared environment. These two important and interesting observations, that the heritability of personality is approximately 40%-60% at the population level and that nongenetic variation in personality is attributable to nonshared environmental factors, are the primary findings from behavioral genetics used to inform conventional personality theories. Yet, they do not do justice to the important developmental patterns in the genetics of personality.

The relative influence of genetic and environmental effects may shift across the lifespan, rather than remain static. Age trends in the heritability of personality have been reported in quantitative syntheses by Kandler (2012) for Neuroticism and Extraversion, and Briley and Tucker-Drob (2014) for all of the Big Five. In both syntheses, age trends have been very similar across each of the Big Five traits. [...] Heritability of personality is estimated at approximately 70% in early childhood, declines to approximately 50% by late adolescence, and subsequently declines to approximately 35% by late adulthood. Nonshared environmentality increases from approximately 30% to 50% to 65% from infancy to late adolescence, to late adulthood. However, at least some of this trend may reflect method bias, as nearly all of the effect sizes for very young children come from parent-reports. These ratings may exaggerate differences between siblings and thus inflate heritability.