Cooperation and competition as a function of zygosity in 7- to 9-year-old twins

A study of 31 monozygotic (MZ) and 24 dizygotic (DZ) Singapore twin pairs ( aged 7.6 to 9.7 years) is reported. MZ and DZ twins differed in cooperative and competitive behaviors measured by within-pair reactions to payoff stru ctures designed to elicit simultaneous cooperation (SCO) or reciprocal altr uism (RA) on a marble-pull task. Reciprocation entailed turn-taking in rewa rds, DZ twins showed greater competition than MZ twins when competing for m arbles in the RA condition, as predicted, Unexpectedly, however, MZ twins w ere markedly more competitive than DZ twins under the SCO condition, when s uccess on the task always rewarded both twins. The competition arose over w hich end of the apparatus was to be used on a given trial. This interaction between condition and zygosity could not be related to differences in pare nts' reported treatment of their children or zygosity beliefs, nor did it a ppear to be related to differences in factors such as time spent together a nd shared activities. An explanation in terms of uncertain or unstable domi nance in MZ twinships is suggested. (C) 1998 Elsevier Science Inc.