A Georgia mother's outrage-inducing video of a school principal paddling her 5-year-old son sheds light on America's enduring — but slowly fading — affinity for corporal punishment.

A majority of parents say they approve of spanking kids, although their numbers are declining, public opinion polls show. Most states have banned it at school. And yet 19 states, primarily in the South and West, still allow it: Alabama, Arizona, Arkansas, Colorado, Florida, Georgia, Idaho, Indiana, Kansas, Kentucky, Louisiana, Mississippi, Missouri, North Carolina, Oklahoma, South Carolina, Tennessee, Texas and Wyoming.

Students in those states received corporal punishment 166,807 times in the 2011-2012 school year, according to the most recent federal data available. That's an average of more than 900 times each school day.

That level of frequency seems to contradict a wide body of social science research on the negative impact of slapping, spanking or paddling a child. Some argue that spanking can help drive down a child's IQ or ability to learn. Others say it triggers more aggressiveness. And child welfare advocates warn that corporal punishment — along with harsh but non-physical discipline — is often applied disproportionately to students with disabilities and those who are black, raising the chances that they will fall behind in school.

An advocacy group called the National Child Protection Center has proclaimed April 30 "Spank Out Day." It's meant to encourage alternatives to corporal punishment.

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So why do so many people still do it?

First, there's no federal prohibition. A teacher's right to smack a kid is protected by the U.S. Supreme Court, which ruled in 1977's Ingraham v. Wright that physical discipline in public schools didn't violate the Constitution's ban on cruel and unusual punishment or its due process protections. In that case, the court cited America's long tradition of corporal punishment in schools — as long as it's not excessive or unreasonable.

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Corporal punishment's strong support in the South seems to be rooted in a number of cultural factors, including a strict interpretation of the Bible, a conservative approach to law and order, and the legacy of using of violence to control people, as in slavery, said David Finkelhor, who heads the Crimes against Children Research Center and Family Research Laboratory at the University of New Hampshire.

At the same time, spanking has disproportionate support among blacks, which, scholars say, reflects in part parents' concerns that they need to keep their children in line at home to protect them from the possibility of violence outside of it. In some black households, corporal punishment is considered morally superior to sparing it.

More than three-dozen countries have banned corporal punishment, part of the spread of human rights advocacy in recent decades. That makes America something of an outlier.

Many traditionalists argue that corporal punishment works in moderation and if done in specific conditions. Among most states that allow corporal punishment in schools, there are strict guidelines — including the requirement that parents to be part of the decision, and that it not cause serious injury. But Finkelhor, along with many other child-development researchers, say that that's misguided, since non-physical discipline has been shown to be more effective.

"My analogy is that if you have two medicines, and one of them you have to be very careful about where you administer it and worry about side effects and negative outcomes, and the other is just as effective, it's a no-brainer which one you should use," Finkelhor said.

Donald Greydanus, the founding chair of the Department of Pediatric and Adolescent Medicine at Western Michigan University, said he's found that many Americans, even when presented with evidence that corporal punishment damages children, prefer to stick with what they're already doing.

"What I’ve noticed is if you have science that says don’t do this but if your personal beliefs, religious beliefs, what your parents taught you is the opposite, a lot of people will go with the opposite rather than the science," Greydanus said.