Washington, D.C. ? Stepfathers make slightly better parents than married biological fathers, researchers found in a new study of at-risk urban families.

Mothers reported that stepfathers were more engaged, more cooperative and shared more responsibility than their biological counterparts did, according to the study, published in this month’s issue of the Journal of Marriage and Family.

Lawrence Berger, the study’s lead author, cautioned that the findings applied only to “fragile families,” defined as low-income urban families prone to nonmarital births.

The findings contradict a popular view among social workers and family policy experts that biological fathers invest more in their own flesh and blood.

“I think this research does, to some extent, call some of those assumptions into question,” said Rebekah Levine Coley, a developmental psychologist at Boston College.

“It certainly shows that stepfathers or even unmarried social fathers can be quite productive in rearing children,” she said.