Co-conspirators and double-dealers: a twin film analysis

Psychological studies using diverse perspectives and procedures find greater cooperation and affiliation between monozygotic (MZ) twins than dizygotic (DZ) twins. The majority of supportive evidence comes from self-reports and parental ratings. The present study compared judgesʹ impressions of MZ (n=34 pairs) and same-sex DZ (n=12 pairs) twin childrensʹ joint behaviors during filmed puzzle completion. Twinsʹ mean age was 8.65 years (S.D.=1.42) and ranged between 6.47 and 12.13 years. The analysis used a multivariate mixed model analysis of variance with Helmert contrasts. MZ twins were rated significantly higher than DZ twins on five measures reflecting cooperation, and significantly lower on one measure reflecting lack of cooperation. Significant differences did not emerge from the contrast between 22 MZ hand-concordant and 12 MZ hand-discordant twin pairs. Results from this variant approach to twin relations concur with findings from more objective assessments. Specifically, they support the view that individuals respond to, rather than create, twin group differences in social relations.