I have read about many attachment studies. This is a topic I am especially interested in as the mom of a Post-Institutionalized child. My web support lists for parents of Post Institutionalized kids have many postings daily of parents who are dealing with RAD or concerned with RAD (all PI kids). Or discussing RAD studies. I am on several weblists, some of which have RAD and language development researchers as regular contributors.
I do NOT agree that there is any any conclusive evidence that shared parenting causes attachment disorder. Studies have been interpreted to support both positions. As a matter of fact, the research I've read about the process of attachment seems to indicate that failing to make the other parent available to "regularly meet the child's basic needs" actually interferes with the normal process of attachment with that parent.
An alternating week schedule, or changing every few days, should not in itself cause attachment disorder. What the researchers seem to agree to be the base cause for attachment disorder is a pattern of the child's caregivers failing to consistantly meet the child's basic needs. Which is why children who were babies in orphanages, in which they were left in wet diapers, seldom held and comforted, who were often left hungry and, because of stressed budgets (no money for heat) and limited staffs, also often cold . Who were not picked up and comforted when they expressed the need. etc. THAT is what causes RAD. And even then, my kiddo, experiencing all that orphanage neglect from birth to 25 months STILL does not suffer from RAD.
I am sorry, but I can not support the assertion that shared custody of young kids causes RAD. If a child has the experience of either available parent being there on a regular basis to meet their basic needs, there is no evidence any attachment disorder should result.