what causes it?
It is based on a problematic history of care and social relationships.
Reactive attachment disorder
reactive attachment disorder is by definition based on a problematic history of care and social relationships. 
what is reactive attachment disorder? It is a severe and relatively uncommon disorder that can affect children.
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Reactive attachment disorder [SEP] Causes
Although increasing numbers of childhood mental health problems are being attributed to genetic defects, reactive attachment disorder is by definition based on a problematic history of care and social relationships. Abuse can occur alongside the required factors, but on its own does not explain attachment disorder. It has been suggested that types of temperament, or constitutional response to the environment, may make some individuals susceptible to the stress of unpredictable or hostile relationships with caregivers in the early years. In the absence of available and responsive caregivers it appears that most children are particularly vulnerable to developing attachment disorders. While similar abnormal parenting may produce the two distinct forms of the disorder, inhibited and disinhibited, studies show that the abuse and neglect was far more prominent and severe in the cases of RAD, disinhibited type.