AbstractThe behavior of interest in this study was participation or non‐participation in a non‐school skill‐development program carried out in Ottawa, Ontario, for children living in a rent‐to‐income housing complex. Participant and non‐participant children differed sharply only in sibship size; the larger the sibship from which a child came the greater the probability that he or she would participate. The basis for this relationship seems to have been a strong tendency for the probability of a child's participating to increase whenever a brother or sister in the same family p