filmed 4-month-old babies in face-to-face interaction with primary caregiver mothers, secondary caregiver fathers and primary caregiver fathers. Primary caregiver fathers, like primary caregiver mothers, spent more time smiling, imitating and holding babies than the secondary caregiver fathers. Smiling, imitating and holding babies are all part of reciprocity and interactional synchrony which, as we saw on page 75, are part of the process of attachment formation