SAN FRANCISCO -- When Jordan Riak of Alamo happens upon a spanking in progress, he moves in fast.

"I intervene. I confront them," said Riak, who runs a Web site called www.nospank.net.

"You're not going to change the behavior of the spanker unless they know they're going to embarrass themselves in public."

Often, though, parents fume and tell Riak to butt out. On the scorched turf of spank vs. don't spank, the middle ground lies fallow.

The debate has ripened lately as conflicting studies show that spanking causes deep, lasting harm or none at all. It swirled inside Moscone Center in San Francisco on Friday, when a UC Berkeley psychologist supplied the latest in a line of research that has some people heralding a comeback for the tush-whack.

Diana Baumrind of Cal's Institute of Human Development studied long-term histories on more than 100 East Bay families and found that occasional spanking does not damage a child's emotional or social development.

After stripping out parents who doled out unusually harsh physical punishment, Baumrind and co-author Elizabeth Owens found that children who were spanked moderately were no worse off than if they were spanked seldom or not at all. Verbal punishment was no better, and sometimes worse.

Baumrind, speaking at the annual convention of the American Psychological Association, defined spanking as "striking the child on the buttocks or extremities with an open hand without inflicting physical injury with the intention to modify behavior."

She distinguishes between impulsive, reactive spanking and spanking as a planned response to misbehavior and part of a child-rearing strategy.

"Although I do not regard spanking as less humane than other forms of punishment, I am not an advocate of spanking," she said. "But a blanket injunction against its use is not warranted by the evidence. It is reliance on physical punishment, not whether or not it is used at all, that is associated with harm to the child."

Baumrind's work has brought her national acclaim among those who hope to bury an anti-spank movement that began 50 years ago, when legendary pediatrician Benjamin Spock first warned of trauma from corporal punishment of children.

The hands-off approach flowered in the permissive 1960s and '70s. Lately, spanking has emerged from the shadows, said Kevin Ryan, director emeritus of the Center for the Advancement of Ethics and Character at Boston University.

"Mother wit is asserting itself," Ryan said. "We as a society are realizing there's something amiss. There's an awful lot of kids that are enormously undisciplined. Sweet reason just takes you so far."

Web sites are abuzz with chat over whether to spank. Although surveys show more than 60 percent of parents condone spanking, their ranks have thinned.

Opponents say there are no clear lines between spanking and abuse; it is a case of degrees. They cite research that links spanking to more extreme abuse, juvenile delinquency, sexual troubles and other problems.

Anti-spanking advocates have focused on the 23 states in which corporal punishment in schools remains legal. California is not among them.

Murray Straus, a sociology professor at the University of New Hampshire, noted that Baumrind does not claim that children who were spanked turned out better.

"It's one of the great American myths that if you don't spank or yell at your kids, they're going to run wild," said Straus, author of "Beating the Devil Out of Them: Corporal Punishment in American Families."

"If there is research, even if it may not be totally conclusive, that spanking has harmful effects, why do it?"

Baumrind's critics say her concept of proper spanking is flawed; some parents may spank with an even keel, but more often it comes out of impulse and anger.

"She's describing a scenario that's a fantasy," said Riak, 66, who in 1999 ran a failed bid for a "no-spank zone" in Oakland. "She's been the darling of spankers. She makes spankers feel good."

Baumrind noted her research dates back to 1968, "when parents didn't mind saying they spanked." Her study looked at largely white, middle-class families in the institute's "longitudinal" database, which contains research from detailed interviews and home visits.

Even in liberal Berkeley, where most of the subjects lived, the study found 4 percent of parents never used physical punishment on their pre-school-age children, and only 16 percent never used it from then until age 8 or 9.

By the time their children were 14 and 15, more than 62 percent of parents used no physical punishment. Even some conservative Christian spanking advocates say corporal punishment on children 11 or older is a bad idea.

Baumrind said her research found no harmful effects even with older children, but she said ages 2 to 6 were the "safest" for spanking.

The American Academy of Pediatrics remains opposed to spanking, calling it "the least effective way to discipline."

Baumrind's work offers a fresh look at a difficult issue for doctors, said Dr. Lawrence Diller, a behavioral pediatrician in Walnut Creek and author of "Running on Ritalin," which looks at use of the popular drug for attention deficit disorder. Diller said he hears every week from parents grappling to control their children.

"There's a sense that if we even touch our kids, we're going to have a visit from a social worker," Diller said.

"We're living a legacy of Freud and Dr. Spock, and I'm handing out Ritalin pills by the shovelful."