Infants Use Physical Size, but Not First Possession, to Predict the Outcome of Territorial Conflict, and Don’t Prefer Owners over Thieves

Conflicts over resources are ubiquitous in social life. Previous cross-species and human research suggests that one way for such conflicts to be resolved is for individuals to respect ‘first possession’ – that is, to cede resources to whoever had them first. Additional research suggests that individuals should prefer owners over thieves as cooperative partners. Here, we tested whether preverbal infants expect possessors to prevail over intruders in territorial conflict, and whether infants evaluate first possessors more positively than thieves. Across six laboratory studies (N = 227), we found positive Bayesian evidence that (a) 10–18-month-old infants do not systematically expect agents to defer to first possessors in territorial conflict, and (b) that 10–13-month-old infants do not preferentially evaluate first possessors over those who attempt to take (or succeed in taking) the possessions of others. In contrast, using identical methods, we found that infants do expect larger, more physically formidable agents to prevail in territorial conflict over smaller intruders. These results demonstrate that first-possession cues alone are not sufficient for regulating conflict expectations, nor evaluations, of ownership in infancy.