A substantisl body of data shows that identical (Monozygotic or MZ) twins maintain closer relationships with one another relative to fraternal (Dizygotic or DZ) twins. The present research concerns the identification and examination of factors underlying social relationships within MZ and DZ twinships. It is particularly concerned with the further application and refinement of observational methods for capturing subtle aspects of cooperation and competition between twins and unrelated children; this interest extends earlier research on social interaction between young twins. Some old questions, such as: are MZ twins more closely affiliated than DZ twins, and why? acquire new significance in light of social-genetic and kinship-genetic explanations of behavior. The present study will determine if twin group differences in cooperation and competition observed in childhood samples are also maintained in an adolescent twin sample. This new approach encourages reevaluation of more traditional theories of twin dynamics, such as those offered by psychoanalytic theorists. The implications of outcomes from studies of twins and non-twins, for social processes and activities within groups and populations, are related areas of interest.