It's a topic that riles up emotions and opinions the way few others do in the contentious world of parenting philosophies. Spanking. Should you? How could you? Is it right? Is it wrong? Online message boards are flooded with messages on the topic: the confessions, the wrath, the full-on support. "Yes, I've done it, even though I always swore I wouldn't," writes one. "Sometimes spanking works best," responds another. And then there are the vocal opponents. "Spanking," writes one, "is abuse."

The spanking wrangle has a long history in scientific research, and new findings to be reported today at the American Psychological Association Summit Conference on Violence and Abuse in Relationships will intensify the debate yet again. In a provocative paper, Murray Straus, co-director of the Family Research Laboratory at the University of New Hampshire, says that spanking kids increases their risk of sexual problems as adults. Straus, a longtime researcher in the field, analyzed four prior studies and found that teens or young adults whose parents used corporal punishment were more likely to coerce dating partners into having sex or to engage in risky or masochistic sex.

One stat: the 25 percent of university students who ranked highest on a corporal punishment scale insisted on sex without a condom, compared with the 12.5 percent of university students who scored lowest on the scale. Another: 75 percent of college students who'd been spanked a lot said they were sexually aroused by masochistic sex, compared with 40 percent of students who were never spanked. "It's so consistent with so many other studies showing harmful side effects," says Straus. "It didn't surprise me."

The new study has its weaknesses, but so does just about every other paper in the field. For starters, you can't study spanking in the randomized double-blind way you can a medication. It would be ethically inappropriate to divide a bunch of kids into two groups, spank some, spare others and then compare how they fare 10 or 20 years down the road. And double-blind? Impossible to disguise spanking in a dummy pill. So there's no way to absolutely prove cause and effect. The study also relies on students' own recollections of their childhood experiences. Straus says he controlled for people covering up mistreatment by their parents. On the other hand, the students could also have exaggerated. "It's possible," says Strauss, "though I don't find it too plausible."

Elizabeth Gershoff, a researcher at University of Michigan's School of Social Work, says Straus's findings are consistent with the literature. "I have every faith in his research," Gershoff says. "The more children are spanked, the more aggressive they are and the more likely they are to engage in delinquent or at-risk behaviors." Sexual behavior is just one example of that behavior, she says. One lesson kids learn, says Gershoff, is that if you have the power in a relationship, you can use aggression to get your way. Another: "[Kids] may learn that sometimes there's pain and fear involved in loving relationships."

Gershoff, who published a large analysis of the spanking research in 2002 and has just completed a new paper about spanking in the context of human rights and public policy, says spanking may work to gets kids' attention, but it doesn't teach them how to behave appropriately in the long-term. A little tap once in a while is going to have minimal risk, but the risk increases "the more you do it and the harder you do it," she says. "I think everything we know from the research is that it doesn't work and it might have negative side effects."

Not every scientist agrees. Robert Larzelere, a human development researcher at Oklahoma State University, says that "conditional" or "backup" spanking in two-to-six-year-old kids can be useful. The spanking needs to be nonabusive (two open-hand swats on the behind from a parent who's not "angrily out of control") and it needs to be used not as a first line of response but as a backup to other kinds of discipline, like timeouts, grounding and reasoning. "Under these conditions, the evidence suggests that it's effective," says Larzelere. Too often, he says, spanking research lumps corporal punishment into one big group, failing to draw the line between overly severe punishment and a couple of taps on the buttocks. His conclusion: conditional spanking isn't more harmful than any other kind of discipline. The key, he says, is that parents need to discriminate between "inappropriate and appropriate use."

No single study is likely to stop the practice of spanking. The Bible says, "He who spares the rod hates his son, but he who loves him is careful to discipline him," and plenty of Americans interpret the passage literally. The Christian group Focus on the Family, for example, says there's no excuse for abusing a child, but spanking is OK when it's done right. "We believe that appropriate, disciplinary spanking, used in the context of a warm, nurturing parent-child relationship, is not abusive or harmful to the emotional well-being of the child," says Bill Maier, Focus on the Family's vice president and psychologist in residence, in an e-mail.

But researchers like Straus, who calls himself a humanist, say children need to be protected. Nobody knows exactly how many people spank their kids, but in one survey Straus found that more than 90 percent of Americans have spanked their toddlers, and while not all will turn out to have dysfunctional sex lives or be aggressive adults, he and others are worried about those who might.

Should there be a policy against it? The American Academy of Pediatrics goes as far as to say that parents should be encouraged to use other methods of discipline. Twenty-two states, meanwhile, still allow spanking in schools. And while there's plenty of grass-roots effort to end it (the group End Physical Punishment of Children, or EPOCH-USA, is holding its annual SpankOut Day on April 30), many Americans are wary of taking too radical a step. When Sally Lieber, an assemblywoman in Silicon Valley, introduced a bill last year that would ban corporal punishment in her state, the public let her know that they didn't want the government messing with their parental rights. "It obviously touched a nerve," says Lieber. "It was like being in the eye of giant cultural hurricane." A hurricane that shows no signs of dissipating.