Another weakness of the role of fathers in attachment is that most research has focused on mother-infant interactions as the primary attachment. Most studies have focused on mother-infant interactions as it is usually the mother who takes on the role of primary caregiver in western societies. Psychologists debate this could be because of traditional gender roles within society, female hormones (e.g. oestrogen) meaning mothers are naturally more nurturing - biologically conditioned to take on the role, fathers take on the role of a 'playmate' rather than a nurturing role.