AbstractNewly hatched Leghorn cockerels (124) wearing hoods containing 8.5° prisms were reared in large or small groups and with high or low ratios of spatially distributed seeds on a sand filler. Only chicks reared in large groups demonstrated negative aftereffects when O° plates were substituted for the prisms on the seventh day. For large groups, high seed density resulted in smaller lateral pecking errors with prism displacement but larger negative aftereffect errors without displacement than did low seed density. For small groups, however, high seed density produced a reversal of negative aftereffect directions. High group density was necessary for negative aftereffect development, probably by its influence on socially mediated pecking rates.