Very interesting. I know that a percentage of children who are adopted months or years after birth, often suffer from a range of attachment disorders, or in rare cases "reactive attachment disorder."
Because the circumstances that lead to adoption, it is not uncommon for infants who are later adopted to be neglected. Children whose mother dies in childbirth, or who are born to drug addicted parents, or simply born to parents who do not want to be parents, or children who are institutionalized shortly after birth, generally are not exposed to the sorts of interactions that children who are loved and wanted are exposed to.
The cooing, the letting the infant grab your finger, the eye contact and snuggling - these are all important to human development. And if an infant does not experience these kinds of interactions, that child may be at risk of future problems interacting with other people. So social skills, and the building blocks for empathy, (along with the ability to integrate sensory information) are among the first things that an infant learns. If a child misses these lessons in infancy, he or she will have gaps unless there are steps taken to integrate these important skills later in life. And like many developmental skills, it is harder to learn later in life. But not impossible.
