Despite the theoretical progress reviewed above, it has been notoriously difficult for behavioral genetic analyses to uncover environments that have an impact on psychological outcomes (Plomin & Daniels, 1987). For example, Turkheimer and Waldron (2000) found that less than 5% of the variance attributed to the nonshared environment could be accounted for by measurable aspects of the environment. This finding led Turkheimer (2000) to affirm the “gloomy prospect” that nonshared environmental effects were too idiosyncratic, complex, or transient to identify with scientific inquiry. Costa and McCrae (2008) have similarly commented that “ambitious attempts to pin down substantive contributions of the non-shared environment have largely failed” (p. 168). Importantly, our findings indicate a substantial proportion of variance in personality traits is influenced by environmental experiences that act to make siblings different from one another that is not attributable to measurement error and is stable across time. Although we take seriously previous failures to “pin down” measurable nonshared environmental experiences, our findings suggest a substantial amount of variance exists in personality traits that is truly environmentally mediated, not random error of measurement,
