Over the course of one decade, a continued observation of monozygotic and dizygotic twins, reared apart after being separated during infancy, sought to clarify the sources of human psychological differences. The study subjected 100 sets of raised-apart twins to 3 days of intensive physical and psychological assessment. In support of previous studies, the 3-day observation concluded an approximate 70, percent association of IQ variance with genetic variance in the separated monozygotic twin population. Other areas of significant similarity between reared-apart monozygotic twins and those reared together include: temperament, personality, occupational interest, and social attitudes.
A consolidated report related to personality is presented in the table below. These findings support the hypothesis that genetic variance affects psychological variance through the indirect influence of the environment surrounding development. Strong genetic influence of psychological and behavioral traits does not diminish the value of propaedeutic intervention including parenting and education.
As environmental differences are more tightly controlled in a given population, the heritability of studied traits in that population is:
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