Previous behavior-genetic research on adult personality relied primarily on self-reports or peer reports that may be subject to contrast effects resulting in biased estimates of genetic and environmental influence. In the German Observational Study of Adult Twins (GOSAT), personality traits of 168 monozygotic (MZ) and 132 dizygotic (DZ) twin pairs were rated on 35 adjective scales, largely markers of the Big Five. The ratings were provided by 120 judges who never met the twins but observed videotaped behaviors of one twin of each pair in one of 15 different settings. The aggregated video-based trait ratings were highly reliable, and substantial correlations were obtained between MZ as well as DZ twins. Model-fitting analyses suggested about 40% genetic, 25% shared environmental and 35% nonshared environmental influence. Extraversion was the only trait that seemed not to be influenced by shared environment.
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