One possible interpretation of this result would be that the investigated families were at greater risk to begin with, and that the investigation helped them to recover to the expected level of risk. But if this were so, the authors write, households with recent investigations would have greater risk than households with more distant investigations. Statistical analysis found no such association. They concluded that Child Protective Services investigations had little or no effect.

The researchers were in some ways unsurprised by their findings. Even when services are offered, they usually take aim at immediate risks — substance abuse, for example, or domestic violence — not abiding problems like poverty or poor social support. Whatever interventions were offered apparently failed to reduce the risk for future child abuse.

Dr. Kristine A. Campbell, the lead author of the study, said that it may be too easy to blame Child Protective Services. “I believe that C.P.S. has a critical role,” she said. “As a pediatrician, when I’m there in the middle of the night with a child who has been beaten up, I need them. But we have to look at other systems that can really create a safety net for these children.”

Other experts agreed. “I don’t see investigation as an intervention. I see it as an activity to gather information,” said Jill Duerr Berrick, a professor of social welfare at the University of California, Berkeley. “While we might all hope that C.P.S. could do more, in the current context it doesn’t. This is an important study that will get people talking again.” Dr. Berrick was not involved in the work.

The authors acknowledge that the study, which appears in the October issue of The Archives of Pediatrics & Adolescent Medicine, has certain weaknesses: some potentially modifiable risk factors — intimate partner violence and substance abuse, for example — were not included in the data they used. And not all of the five different geographical sites systematically collected information on all risk factors.