Can Adoption Lead to Child Abuse?

At Huffington Post, Lisa Belkin answers "yes:"

I have not been able to get four-year-old Sean Paddock, or 11-year-old Hanna Williams, or 7-year-old Lydia Schatz out of my mind. As Erik Eckholm reported in the New York Times yesterday, and Anderson Cooper discussed on CNN, most recently last week, the three children all died within the past five years, and they had several chilling factors in common.

Each of their deaths were brutal and agonizing: Sean suffocated; Hana, who was found lying naked in the muddy yard, died of hypothermia and malnutrition; Lydia showed signs of a brutal beating. In each case, one or both of their parents has been charged with their murder.

And in each case, those parents are said to have essentially punished their children to death, allegedly because they believed it was God's will. They are said to have been guided by the book To Train Up A Child, by Michael and Debi Pearl, which advocates beating children with rubber tubing, leaving them outside in the cold, and witholding food for days at a time in keeping with Biblical teachings.

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[E]ach of these children joined these families through adoption. Sean was born in the US, as were his five adopted siblings. Hana was from Ethiopia, as was her adopted brother (their parents had six biological children as well), and Lydia was from Liberia (there were two other adopted siblings among the family's nine children).
Is this merely grisly coincidence? Or is there something about the adoption dynamic that makes violent abuse more likely?
One possibility is that adoptive children -- particularly those who spend their earliest years in an orphanage or shuttling from one foster caregiver to the next -- are more likely to suffer reactive attachment disorder, which are essentially the inability not only to bond, but to feel. The effects are not just psychological, but also physical, with evidence these children can have elevated levels of the hormone cortisol, which increases their tolerance for pain. Some speculate that spanking a child with Reactive Attachment Disorder can spiral out of control quickly, because it takes abusive levels of pain before the child actually feels it and responds.

Belkin also notes that a commenter claiming to know Hana's family wrote:

They expected Hana and her little brother to assimilate into their family, and most likely ignored their culture, how they had grown up (customs, beliefs, etc), and most importantly, the trauma that Hana and her brother had gone through in their childhoods. These kids just weren't acting like their biological children. Instead of taking a step back and getting professional help, they decided that they would continue to follow the Pearl method, but continued to up the ante, because these kids were NOT succumbing to being "broken".

When we read media reports of adoptive parents who have abused and/or murdered their children, it raises lots of questions.  Who commits more abuse, biological or adoptive parents?  Is abuse of adopted children because of or in spite of adoption? If adoption is causitive, in what way is it causitive? Are adoptive parents less able to "feel the pain" of a child not biologically related to them? Is that tendency (if it exists) exacerbated if the adoption is transracial? Does RAD and the reaction of the child play a role, as Belkin suggests?  Is the stress of adoptive parenting a factor? Is the connection between adoption and religion implicated?

I wish I had the answers. . . .  What do you think of Belkin's suggestion? Reactions?