Being adopted can be one of the best things to happen to a kid. People who adopt tend to be wealthier than other parents, both because of self-selection and because of the adoption screening process. Adoptive parents tend to be better-educated and put more effort into raising their kids, as measured by things like eating family meals together, providing the child with books, and getting involved in their schools.

And yet, as rated by their teachers and tests, adopted children tend to have worse behavioral and academic outcomes in kindergarten and first grade than birth children do, according to a new research brief from the Institute for Family Studies written by psychologist Nicholas Zill.

Problem Behavior in Kindergarten and First Grade

Institute for Family Studies

As measured by their teachers, young adoptive children were more likely than biological ones to get angry easily and to fight with other students. If a 50 percent score represents an average level of this type of “problem behavior,” adopted kindergarteners were higher than average, at 64 percent, while children with two biological parents were at 44 percent. Children in single-parent, step, and foster families all had fewer behavioral issues than adopted kindergarteners, at 58 percent, although this difference was not significant. A similar pattern (63 percent versus 43 percent) emerged for adopted and biological first graders. For his research, Zill examined a longitudinal study of 19,000 students that was conducted by the National Center for Education Statistics beginning in 1998. Zill is the former head of the Child and Family Study Area at Westat, a social-science research corporation.

Math Skills in Kindergarten and First Grade

Institute for Family Studies

Teachers also said adopted children were less likely to pay attention in class, were less eager to learn new things, and didn't persist as long on challenging tasks. The adopted kids also performed worse on an objective math test and slightly, though not significantly, worse than birth children on a reading test.

Adoptive parents go to great lengths to do a great service. Why are their young kids' behavior and test scores nonetheless worse, on average?

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. That adult can be the adoptive parent, but the adoption itself might mean that the bond with the birth parent was disrupted or never formed, Zill writes. In the worst cases, these children might have experienced a traumatic event prior to their adoption. Early trauma can affect the parts of the brain that control mood and learning.

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.

Zill makes it clear that this is not an argument against adoption or adoptive parents. Adoption is generally considered a better solution than institutionalization, remaining in a bad home environment, and in some cases even foster care. If anything, Zill's research underscores the importance of a strong attachment between parents and children in early childhood. Unfortunately, the challenges faced by adopted children are just one visible example of that.