It seems dads are a lot looser than moms in what they let toddlers do in exploring their environments. Moms create attachment. Dads create activation, this researcher says, because they are less protective. They measured this by how far the parent stayed from a 12- to 18-month old baby while it crawled up some stairs to get a toy. The optimum distance for the baby to be adventurous but self-confident was about arm's length for dads.
This researcher is putting "activation theory" on the same level as attachment theory. He says dads "activate more exploratory behavior." I believe the latter. I'll have to think about the former.