To spank or not to spank? This age-old parenting question elicits fierce debate among parents, psychologists and pediatricians. Surveys suggest that nearly half of U.S. parents have spanked their children as a disciplinary tactic, but many experts argue that this form of punishment—hitting a child on the bottom with an open hand—increases the risk that kids will develop emotional and behavioral problems. Other scientists counter that research on the issue is fraught with problems, making it impossible to draw definitive conclusions. A new meta-analysis speaks to several of the most contentious points in the debate and concludes that spanking does pose risks, but differences of opinion persist.

In the meta-analysis, researchers Elizabeth Gershoff and Andrew Grogan-Kaylor of the University of Texas at Austin and the University of Michigan, respectively, evaluated 75 published studies on the relation between spanking by parents and various behavioral, emotional, cognitive and physical outcomes among their kids. They found that spanking was associated with 13 out of a total of 17 negative outcomes they assessed, including increased aggression, behavioral and mental health problems, and reduced cognitive ability and self-esteem.

This was not simply an attempt to synthesize studies—Gershoff and Grogan-Kaylor also wanted to address two concerns often raised about the body of research linking spanking to childhood problems. The first is that much of it has evaluated the effects of physical punishment in general, without homing in on the effects of spanking specifically—and because physical punishment can include tactics such as hitting with objects, pinching and biting, this “lumping problem” may ultimately exaggerate spanking's risks. The second concern is that many published studies are “cross sectional,” which means that they evaluate the effects of spanking by collecting data at a single point in time, making determinations of cause and effect difficult. A cross-sectional study might, for instance, find that aggressive 10-year-olds were more likely than docile 10-year-olds to have been spanked as toddlers, but that does not mean that spanking made them aggressive. They may have been spanked because they were acting out back then, too.

To confront these issues, Gershoff and Grogan-Kaylor did several things. First, they limited their meta-analysis to studies that evaluated the effects of spanking, slapping and hitting children without the use of objects and found that spanking is still associated with negative outcomes. They also compared the results from cross-sectional studies with results from longitudinal studies, which track the kids' behavior over time and are better able to tease out cause and effect. Gershoff and Grogan-Kaylor found that spanking is associated with negative outcomes in both types of studies, which strengthens the argument that spanking poses risks.

Yet some researchers remain skeptical. Studies suggest, for instance, that the effects of spanking can differ depending on the circumstances. Two studies have found no associations between spanking and mental health problems among kids who were spanked less than once or twice a month; other research has shown that spanking has much less of a negative effect on preschool kids than on infants and adolescents. So the conclusion from the meta-analysis that spanking itself is dangerous might be overly simplistic. “I think it's irresponsible to make exclusive statements one way or another,” says Christopher Ferguson, a psychologist at Stetson University in Florida.

And then there is the chicken-or-egg question: Are kids spanked because they act out, or do they act out because they are spanked—or both? Ferguson tried to control for the effects of preexisting child behavior in a 2013 meta-analysis he published of the longitudinal studies on this issue; when he did, the relation between spanking and mental health problems was much smaller than it had appeared without these controls in place. As a further demonstration of the importance of careful statistical controls, Robert Larzelere, a psychologist at Oklahoma State University, and his colleagues reported in a 2010 study that grounding and also psychotherapy are linked just as strongly to bad behavior as spanking is but that all the associations disappear once controls are used.

Still, a number of individual studies have found associations between spanking and negative outcomes, even after controlling for preexisting behaviors. Thus, Gershoff says that in spite of the lingering controversy, the safest approach parents can take is not to spank their kids. “Studies continue to find that spanking predicts negative behavior changes—there are no studies showing that kids improve,” she says. In other words, not a shred of data suggests spanking actually helps kids become better adjusted—and with the large body of work suggesting it might do harm, why take the chance?

This story originally ran online with the title “What Science Says—and Doesn’t—about Spanking.”