AbstractIn an earlier study we found that shock‐elicited defensive aggression was intensified in rats that had been deprived of playfighting as juveniles. The three experiments reported here extend this phenomenon to the more naturalistic intruder/resident paradigm for eliciting defense. Rats were reared from 20 to 50 days in one of three conditions: in pairs or in isolation with or without 1 hour of daily playfighting experience. They were rehoused in small groups at 50 days, when the frequency of play is beginning to wane, in order to eliminate the effects of ongoing isolation at the time of testing. They were tested for defense at 80 to 100 days by being placed in a resident's cage for 10 minutes.Our main finding was that play‐deprived animals spent significantly more time immobile after they had been attacked than did animals of the other two groups. The increased immobility associated with playfighting deprivation is not caused by baseline differences in emotionality such as those elicited by a novel environment (Experiment 1), the presence of a strange animal (Experiment 2), or nonsocial aversive stimuli (Experiment 3). Furthermore, play‐deprived rats were not more reactive when pinched with forceps to stimulate a bite delivered by a conspecific, whether or not another rat was present behind a divider. Thus isolates' greater reactivity may be restricted to situations involving pain coupled with close proximity to and contact with another rat.A secondary finding was that there were no differences in defensive behaviors other than immobility. The appropriate generalization to be drawn from these studies is that early social deprivation facilitates the defensive response to a social threat that happens to be prepotent under the given experimental condi