Parents have been told for years that high-quality day care did no harm and may even provide added benefits for their pre-school children compared with staying at home. So the results of a large study released last week that finds a tendency for young children who spend more than 30 hours a week in day care to be more disobedient and aggressive are disturbing. The absence, however, of a likely cause of the misbehavior leaves parents and day care personnel alike little more enlightened than before the conclusions were released.

Children who spend long hours in day care, according to the study of 1,300 children funded by the National Institutes of Health, are more likely to engage in defiant behavior or to bully other children once they get to kindergarten. Among those who spent more than 30 hours a week in day care, 17 percent fell into these categories, while only 6 percent of those who spent less than 10 hours a week did. A higher quality of care had only a slight improvement on the children’s behavior.

Certainly, with 83 percent of the children who spend long hours in day care not showing more aggressive behavior, it is difficult to conclude that day care alone is responsible for producing the nasty dispositions. In news reports, experts not connected with the study have pointed out that the work may merely be reflecting the fact that parents of unruly kids are more likely to keep them in day care for longer hours or that children who spend lots of time in day care may come from homes of high stress and are reacting to that stress.

The study itself doesn’t speculate on a cause, but with both parents being employed now the norm, it would be worth finding out. The addition of even one or two unruly children to a kindergarten classroom can interrupt learning for all pupils as well as make the teacher think about early retirement.

Finding the cause will be difficult to do in research that relies more on observation than controlled experiments. Without it, however, there is little families or day care centers can do to improve behavior.