19 November -  Nancy Segal Cal State Fullerton Psychology Twin and Adoption Research
Tests of Evolutionary-Based Hypotheses     Twin studies are a vital tool for exploring hypotheses generated by behavioral-genetic theory.  They have, however, been used less frequently from the perspective of evolutionary psychology. The ways in which twin research methodology can be integrated into these disciplines will be illuminated with reference to recent and ongoing studies. For example, the nature and bases of human social relationships have fascinated the scientific community for years. Using a "twins-as couples" approach, greater cooperation and affiliation have been observed within MZ than DZ twinships, underlining the contribution of the genotypes of the interactants to social outcomes and processes. This finding is exemplified in a 1999 study of the Prisoner’s Dilemma in which more frequent cooperative trials occurred between MZ than DZ twin adolescents and adults. More compelling, in a study underway, MZ twins reared apart have indicated greater closeness and familiarity following reunion, relative to DZ twins reared apart. These various findings are consistent with Hamilton’s theory of inclusive fitness. Studying twins whose cotwins are deceased offers a different approach to this same class of questions. Recent work shows more severe bereavement response among surviving MZ than DZ twins. MZ twins also show a smaller reduction in grief over time, further demonstrating the greater intensity of their reaction. Evolutionary reasoning suggests that losing a close relative represents a loss in future genetic representation.
    Studies of a novel "twin-like" research design offer a more instructive alternative to the ordinary adoption method. Virtual twins (VTs) are unrelated siblings of the same age adopted into the same family at nearly the same time.  These unique sibling pairs replay essential features of twinship, enabling sensitive tests of shared environmental influence on behavioral development. This project has been ongoing at California State University, Fullerton since1983. New analyses, based on a sample of 90 pairs, yielded an IQ intraclass correlation of .26 (p< .01). This correlation, while statistically significant, falls considerably below correlations of .86, .60 and .50 reported for monozygotic (MZ) twins, dizygotic (DZ) twins and full siblings, respectively.  The VT IQ subtest profile correlation of .08 also falls below corresponding MZ (.45) and DZ (.24) twin correlations. These results are consistent with explanatory models of intelligence that include genetic factors, demonstrating that shared environments have modest influence on intellectual development.   Future plans for this ongoing study include comparing social relatedness across a broad range of genetically and environmentally informative relationships. It will, for example, be important to include VT pairs in studies of cooperation and competition for comparison with MZ and DZ twins, thus extending the body of twin-based research in this domain.