The traditional parenting approach has been to identify the mother as the primary caregiver for kids, giving the father a more authoritative and rolemodeling status. This is also supported by scholarly evidence (eg Unemura et al, Do toddlers prefer the primary caregiver or the parent with whom they feel more secure? The role of toddler emotion).
While in united families this emphasis on the mother is less formal, after divorce there is an explicit identification of who is the primary caregiver (even if there are cases of joint custody).
My question is simply: is there any evidence that the mother is better as primary caregiver after the kids have finished the nursing phase?