WHY CHILDREN FROM THE SAME FAMILY ARE SO DIFFERENT FROM ONE ANOTHER -A DARWINIAN NOTE

The well-established finding that siblings growing up in the same fami ly turn out to be very different from one another has puzzled psycholo gists and behavior geneticists alike. In this theoretical note we desc ribe the possible ontogeny and phylogeny of a sibling differentiation mechanism. We suggest that sibling competition for parental investment results in sibling differentiation on a number of characteristics, pr oducing different developmental trajectories within families. Variatio ns in developmental trajectories within families may have had fitness advantages in ancestral environments because (a) sibling competition f or extrafamilial resources would be reduced and (b) these variations w ould be suited to environments containing a variety of niches or to ch anging environments. Predictions derived from this model and an exampl e of an application to attachment theory are presented.