When it comes to raising children, no debate is more contentious — or longer-running — than the dispute over spanking.

The question of whether a swat on the behind is an acceptable method of discipline has bedeviled parents, divided pediatricians and spawned impassioned anti-spanking and pro-spanking camps.

Opponents like Murray Straus of the University of New Hampshire blame spanking for ills including depression, juvenile delinquency, spousal abuse and lowered mental ability. Tough-love advocates like James Dobson, a conservative Christian psychologist, advise that "the spanking should be of sufficient magnitude to cause the child to cry genuinely."

And a talk given Friday at the annual meeting of the American Psychological Association in San Francisco is likely to amplify the uproar by several decibels. In it, Diana Baumrind, a child development expert at the University of California at Berkeley, asserted that social scientists have overstepped the evidence in claiming that spanking causes lasting harm to the child.

"The scientific case against the use of normative physical punishment is a leaky dike, not a solid edifice," she said.

Baumrind, a psychologist known for her widely respected studies of authoritative, authoritarian and permissive styles of child rearing, said she did not advocate spanking. But she argues that an occasional swat, when delivered in the context of good child rearing, has not been shown to do any harm.

The studies cited by opponents of corporal punishment, Baumrind said, often do not adequately distinguish the effects of spanking as practiced by nonabusive parents from the impact of severe physical punishment and abuse.

Nor do they consider other factors that might account for problems later in life, like whether parents are rejecting or whether defiant or aggressive children might be more likely to be spanked in the first place.

Baumrind described findings from her own research, an analysis of data from a long-term study of more than 100 families, indicating that mild to moderate spanking had no detrimental effects when such confounding influences were separated out. The study drew upon data from the Family Socialization and Developmental Competence Project, which followed families in the Berkeley area over 12 years, from the time their children were preschoolers until they were adolescents.

Straus, who attended Baumrind's talk, praised her study but said the findings did not change his view that spanking is harmful. "There is not absolutely conclusive evidence but there is very strong evidence, and there's strong evidence that other methods work just as well," said Straus.