initial attachment research from Bowlby suggested that infants have a biological drive to form attachments to one main caregiver
later research by Schaffer and Emerson suggested infants can form multiple attachments, particularly to father
had economic implications in terms of 'stay at home mothers' who have been out of workplace
however, not psychological research has focussed on high quality substitute care, it's more common for women to go back to work and for father to stay-at-home and women to be higher earners