The reports add to a growing chorus of voices that challenge the popular Pollyannaism around adoption. Recent reporting by Kathryn Joyce and others have raised questions about coercive and dishonest treatments of birth families in international adoptions. A few particularly bad endings have made national news, including novelist Joyce Maynard’s decision to give her two adopted Ethiopian girls to another family in 2012 and the Tennessee mother who put her adopted 7-year-old son back on a plane to his home country of Russia. According to statistics from the federal Child Welfare Information Gateway, up to 10 percent of adoptions—and very possibly more—end in “disruption,” meaning the adoption process is halted after the child is placed in the home but before the paperwork is finalized.

At first glance, Zill’s findings are mystifying. Adoptive parents tend to be wealthier and better educated than other parents; previous research has found that they are likelier to read to their children and to send them to private schools. This kind of investment is not surprising, since they apply extraordinary resources, time, and patience simply to become parents; by definition, they want the job.

It’s important to note that most of the conditions adopted children are dealing with are things like attention-deficit disorder and learning disabilities, rather than severe intellectual or physical disabilities. And Zill is careful to note that adoptive parents tend to be especially sensitive about their children’s well-being, and aggressive in obtaining diagnoses and related treatment for them. In other words, the very qualities that make adoptive parents stand out—their resources, their proactivity—also prompt them to seek out expert care at the earliest sign of trouble. Another caveat: The sample size of adopted children within the large longitudinal study was 86, so Zill’s analysis has relatively high margins of error.

Still, the report is discouraging on its face. With parents this dedicated, why do adopted children seem to struggle so much? Writing about the first part of the report in October, Olga Khazan noted:

One clue might be attachment theory, which holds that a strong bond with at least one nurturing adult—usually the mother—is essential to a child thriving ... Infants and toddlers with a so-called “disorganized attachment” to their earliest caregivers—those who feel frightened of or dissociated from their parents—are more psychologically vulnerable later in life. Among other things, they have more problems regulating their emotions and managing conflicts without resorting to hostility. Parents who create disorganized attachment with their kids might be the sorts of parents who get their kids taken away and adopted out.

This is surely a significant part of the issue. But defying conventional wisdom, Zill’s new report also suggests no difference between children adopted in infancy and those adopted later in life. In the earlier report on kindergarteners and first graders, children adopted within the first year of life had fewer problems than those adopted later. But a substantial proportion of those not diagnosed by kindergarten received diagnoses later. In the new analysis of older children, there was no statistical difference between the two groups. The new report floats maternal substance abuse in pregnancy as another possible explanation, but emphasizes that it’s mere speculation, because in many cases too little information is known about birth parents.