Infant twins' social interactions with caregivers and same-age siblings.

The study of twin behavior offers the opportunity to study differential patterns of social and communicative interactions in a context where the adult partner and same-age peer are equally familiar. We investigated the development of social engagement, communicative gestures, and imitation in 7- to 25-month-old twins. Twin dyads (N=20 pairs) participated in 10-min, semi-structured play sessions, with the mother seated in a chair completing paperwork for half the session, and on the floor with her children for the other half.

The key to cultural innovation lies in the group dynamic rather than in the individual mind.

Vaesen infers unique properties of mind from the appearance of specific cultural innovation--a correlation without causal direction. Shifts in habitat, population density, and group dynamics are the only independently verifiable incentives for changes in cultural practices. The transition from Acheulean to Late Stone Age technologies requires that we consider how population and social dynamics affect cultural innovation and mental function.