The sibling relationship, as any parent with two or more children knows, is an extraordinarily intense one: young brothers and sisters love and hate, play and fight, tease and mock each other with a devastating lack of inhibition.

Why do some siblings get along harmoniously and affectionately, while others constantly squabble? To what extent are parents responsible for differences in siblings' personalities, and how can they ease the tensions?

In this timely and unusual glimpse into the world of the child, Judy Dunn argues that in fighting, bullying, or comforting, very young...