Infants and Attachment

Adoption and attachment disorders can go hand in hand. I hear lots of stories about adoptees who suffer from RAD and related disorders. My question is, is this common only among children that were adopted when they were older, or is it common in infants too? It doesn't make sense to me that an infant would suffer from an attachment disorder, since they would simply attach to their adoptive parents as caregivers, since they have no memory of their biological parents. But I still see a lot of concerns about it. Does anyone have more information on this?