SUMMARYTwo groups of boys made a series of 200 decisions in a non‐zero sum game called the Prisoners' Dilemma. The boys in the experimental group were selected from a children's home, and the control group from normal families; institutionalisation was the sole differential dimension.A stooge played three different pre‐determined strategies against the naive boys. The stooge either replicated, was more benevolent, or more malevolent, than the naive boy. The results showed an overall higher level of co‐operation in the “own home” boys as compared to the “institutionalised” boys, especially when the stooge played a benevolent strategy. Under all strategies, the “institutionalised” boys showed marked resistance to change from competitive to co‐operative choices. They were often unable to cope with a situation when there was a disagreement between the strategic context, and the individual choice of die partner. Also the “institutionalised” boys were less easily taken in than the “own home” boys by the seemingly co‐oper