According to kin selection theory, indirect reproductive advantages may induce individuals to care for others with whom they share genes by common descent, and the amount of care, including self-sacrifice, will increase with the proportion of genes shared. Twins represent a natural situation in which this hypothesis can be tested. Twin pairs experience the same early environment because they were born and raised at the same time and in the same family but their genetic relatedness differs depending on zygosity. We (...) compared the degree of willingness to fight and sacrifice for the co-twin among monozygotic and dizygotic pairs in a sample of 1443 same-sex and opposite-sex twins. We also analyzed the effect of the subject’s gender and that of the co-twin on those altruistic behaviors. Results partly supported the postulated explanation. MZ twins were significantly more likely than DZ twins to self-sacrifice for their co-twins, but zygosity did not affect willingness to fight for him/her. The genders of the subject and of the co-twin, not genetic relatedness, were the best predictors of aggressive altruistic intentions. (shrink)

Relationships of physical resemblance to personality similarity and social affiliation have generated considerable discussion among behavioral science researchers. A “twin-like” experimental design explores associations among resemblance in appearance, the Big Five personality traits, self-esteem, and social attraction within an evolutionary framework. The Personality for Professionals Inventory, NEO/NEO-FFI-3, Rosenberg Self-Esteem Scale, and a Social Relationship Survey were variously completed by 45 U-LA pairs, identified from the “I’m Not a Look-Alike” project, Mentorn Media, and personal referrals. The mean U-LA intraclass correlations were (...) negligible for all Big Five personality traits on the PfPI and NEO/NEO-FFI-3. In contrast, mean ri values of.53 and.15 for monozygotic and dizygotic reared-apart twins, respectively, have been reported for these personality measures. The U-LA self-esteem correlation was also below the correlations reported for MZ and DZ reared-together twins. Finally, far fewer U-LAs expressed close social relationships than MZA and DZA twins. The present study extends earlier findings indicating that appearance is not meaningfully related to personality similarity and social relatedness. The criticism that MZ twins are alike in personality because their matched looks invite similar treatment by others is refuted. A more judicious interpretation is reactive genotype-environment correlation, namely that MZ twins’ similar personalities evoke similar reactions from others. MZ twins’ close social relations most likely derive from their perceptions of genetically based within-pair similarities that are lacking in U-LAs. (shrink)