Abstract
The present study examined the possible effects of chorionicity of twins on variation of prosocial behavior in the classical twin design. Mothers of 56 pairs of monochorionic monozygotic (MCMZ) twins, 34 pairs of dichorionic monozygotic (DCMZ) twins, and 316 pairs of dizygotic (DZ) twins rated their children's prosocial behavior. The MCMZ correlation for prosocial behavior was similar to the DCMZ correlation (.63 vs. 61), but both correlations were higher than the DZ correlation (.33). Models incorporating the chorion effects were fit to the data. Genetic, shared and nonshared environmental, and chorion effects in the full model were, respectively, 53% (95% CI: 1-70%), 7% (95% CI: 0-37%) , 40% (95% CI: 29-54%), and 0% (95% CI: 0-27%), with the effects of chorion and shared environment being nonsignificant. These findings indicate that genetic and environmental factors in prosocial behavior estimated from twin studies are not significantly influenced by the chorion type of MZ twins.
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