Almost all behavior-genetic studies on adult personality used self-report measures. Although self-reports are valid measures of personality characteristics, perceiver effects and contrast effects may severely distort behavior-genetic analyses. In this paper, we review 12 twin and adoption studies using observational measures of infants’ or children’s personality. As a consequence of small sample sizes in most of these studies, the standard errors of parameter estimates are large and there is little power to detect genetic or shared environmental effects. Although the results of the observational studies are diverse, they suggest effects of the shared environment more frequently than self-report studies. We report results from our German Observational Study of Adult Twins (GOSAT) on the genetic and environmental sources of adult behavior. Participants were 169 MZ and 131 DZ twin pairs of both sexes. We focus on on-line behavior counts that were registered repeatedly in several situations and required a minimal amount of inferences on the part of an observer. Multivariate analyses are consistent with the hypothesis that genes tend to affect observed behavior at a more global level whereas environmental influences are the only source of variance at a more specific level.
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