The extent to which behaviour is determined by ‘nurture’ as opposed to ‘nature’ in mammals is controversial, although most recent interest has focused on genetic determinants. Here we investigate maternal influences on behavioural development by using the approach of cross-fostering between sheep and goats, which, like ourselves, form close individual attachment bonds with their offspring. We show that the emotional bond between a mother and her male offspring, rather than other social or genetic factors, may irreversibly determine these species' social and sexual preferences. Maternal influences on female offspring are weaker and totally reversible. In both sexes, visual cues from the face are important for determining attraction.