Parents, you’ve heard that “spanking your kids probably isn’t the greatest use of a teachable moment,” said Maggie Mertens in NPR’s Health Blog. Well, it can also knock five points off your child’s IQ, according to two new studies by University of New Hampshire sociologist Murray Straus. The effect is strongest in 2- to 4-year-olds, and the spanking-IQ correlation holds up both in the U.S. and in the 32 other countries Straus studied.

"Less-than-ideal practice? Maybe," said Paula Spencer in Woman's Day. "The ruin of society? I don't buy it. Never have. A whole generation of parents was spanked, and most of us turned out okay."

“How might getting spanked on the butt actually affect the workings of the brain?” said John Cloud in Time. Straus says it has to do with “fright and stress,” which make it harder to focus and learn. He could have it backward, of course—maybe kids with “poor reasoning skills misbehave more often”—but other studies back Straus’ interpretation of the data.

The evidence seems pretty conclusive, said Salynn Boyles in WedMD Health News. In a 2002 survey of 88 spanking studies, 90 percent found spanking to be detrimental, raising incidents of aggression, anti-social behavior, and mental-health issues. Critics point out that Straus and other spanking researchers “strongly oppose the practice,” tainting the data, but Straus notes that even if it only has a small effect on IQ, it’s one that parents can control.