COOPERATION AND COMPETITION AS A FUNCTION OF ZYGOSITY IN 7 TO 9 YEAR OLD SINGAPORE TWINS

Abstract

This study focused on the question of whether there is a clear genetic basis for the human social behaviours "cooperation" and "competition". Cooperation and competition were hypothesised to vary with genetic relatedness between the people engaging in these behaviours. A classic twin study, with young monozygotic (MZ) and dizygotic (DZ) twin pairs (age 7.62 to 9.65 years), was employed as the design. MZ twin pairs are identical in genetic makeup while DZ twin pairs share only an average of 50% of genetic material. Zygosity, therefore, represented the independent variable of genetic relatedness. MZ and DZ twins were shown to differ in cooperative and competitive behaviours measured by their reactions to different payoff structures, Simultaneous Cooperation (SCO) and Reciprocal Altruism (RA), on a task chosen for use in this study (Marble-Pull Task). In the Marble-Pull task, rewards can be gained only by cooperating. Competition is not beneficial for either player. Unexpectedly, MZ twins were shown to be significantly more competitive under the SCO condition (or dyad reward condition) than DZ twins. The result of the RA condition (or individual reward condition) was predicted by the hypothesis that DZ twins would show greater competition than MZ twins. Explanations based on environmental influences on the different twin types, e.g. parental treatment, shared activities and amount of time spent together were studied and rejected. The results were discussed in relation to genetic relatedness and issues of unresolved dominance in twinships. It is hypothesized that MZ twins were concerned with unresolved issues of dominance and tried to resolve these issues by competing under a nonthreatening environment offered by the SCO setup whereby there could be no real difference in quantity of reward gained by each co-twin. When there is threat of inequality in terms of actual reward distribution (in the RA condition), MZ twins put issues of dominance aside and behaved more cooperatively than DZ twins. DZ twin pairs are hypothesized to maintain more stable dominance-submission statuses, and were not very concerned with issues of dominance in ordinary, nonthreatening situations. As a result, DZ twins were predictably cooperative in SCO. DZ twins were, however, competitive in RA when there was opportunity for the under-dog twin to assert himself or herself.