Last week, NFL running back Adrian Peterson was indicted for child abuse, and has been indefinitely barred from playing for his team, the Minnesota Vikings. It's since emerged that Peterson is accused of beating his four-year-old son with a switch, leaving welts and bruises that were visible on his skin days afterward.

kids who are physically punished are more likely to suffer from addiction and depression as adults

According to the doctor who examined the son, Peterson's actions constituted child abuse. To this, Peterson apologists have generally replied with one of two arguments: "I was beaten like that, and I turned out fine," or "Peterson overdid it, but he's not a child abuser."

The truth, however, isn't that this sort of punishment is okay, or that Peterson just barely crossed an ill-defined line. Research consistently tells us that any form of corporal punishment — including spanking — is an ineffective way to punish kids. But more importantly, it causes real long-term mental damage: kids who are physically punished, data shows, are more likely to suffer from addiction, depression, and other mental health problems as adults.

This is one reason why 37 countries have explicitly banned all physical punishment of children — even by parents — since 1979. It's not that Peterson overdid it — it's that it's simply never okay to hit a child in the first place.

Physical punishment isn't effective

There's a reason why the American Academy of Pediatrics — along with most leading psychologists — strongly recommend that parents do not even spank their children: research shows it's an especially ineffective form of punishment.

Corporal punishment may temporarily force a child to stop doing something wrong, but it's not a long term solution. "Hitting a child only results in fear and obedience," Karin Österman, a developmental psychologist who's conducted research into the effects of physical punishment, told me when I wrote about spanking in June. "It does not enhance the child’s understanding of why a certain behavior is undesirable."

It also communicates that violence is an appropriate way of dealing with a problem. As a result, research shows, children that experience it become more likely to get into fights with peers and engage in antisocial behavior. In short, violence begets violence.

Physical punishment causes real, long-term damage

Researchers have long known that severe child abuse leads to long-term mental problems. Obviously, the physical pain kids experience immediately makes abuse unacceptable, but the way it makes them more likely to suffer from depression, PTSD, addiction problems, and other mental disorders as adults is equally troubling.

Milder forms of corporal punishment, such as spanking, may not cause the same sort of instant physical harm — but research increasingly indicates they lead to the same type of long-term mental damage.

the difference between severe beatings and milder ones isn't of kind, but of degree

There's evidence that when children are spanked, their cognitive development slows. Later on, as adults, they're more prone to suffering from mood disorders and addiction problems and more likely to commit crimes or abuse their spouses or children. They're more likely to attempt suicide. All of these correlations persist when researchers control for things like socioeconomic status, level of education, and parental support.

This might come as a surprise, but it makes sense in the context of physical child abuse as a whole. The longest-lasting problems caused by abuse are generally mental, not physical.

The difference between severe beatings and lighter ones isn't of kind, but of degree. This is true both for how the punishment is carried out, but also how it affects the developing brain.

"The link between child abuse and negative health effects in adulthood has long been known," said Österman, who studied the long-term effects of Finland's 1983 ban on spanking. "Our study shows that adults who were victims of physical punishment during childhood suffer the same types of symptoms in adulthood."

Many countries ban all physical punishment of children

In the US, during the early and mid 20th century, laws were gradually passed that protected children from physical and sexual abuse, along with neglect. But we're still behind much of the world when it comes to legislation that protects children from all forms of physical punishment. Currently, 37 countries have such laws on the books, with 29 enacting bans since 2000.

Countries with laws that ban all physical punishment of children

In the US, some states have recognized that physical punishment is unacceptable — but only when it comes at the hand of someone other than a parent. 30 states have banned all sorts of physical punishment in schools.

States that ban all forms of physical punishment in school (shown in blue):

For comparison, in addition to the 24 nations in Europe that have banned physical punishment entirely, every single European country has banned it in school.

International law has also moved in the direction of banning all forms of physical punishment of children. 194 countries have signed the UN Convention on the Rights of the Child, a treaty that commits them to "protect the child from all forms of physical or mental violence." In 2007, the UN committee explicitly interpreted this as forbidding physical all punishment. There are three UN states, though, that still haven't ratified it: Somalia, South Sudan and the US.

Why physical punishment is never okay

Despite all this, about 81 percent of American adults feel that spanking is sometimes necessary to properly discipline a child. The most common argument: "I got spanked as a child, and I turned out fine."

But the link between corporal punishment and long-term mental problems doesn't mean everyone who gets punished this way will suffer them — just that they increase the chance of them as a whole. Given that physical punishment also doesn't work in the short term, it makes sense to err on the side of the child.

whether punishment leaves a mark on the skin or the brain is unimportant

Others argue that outside parties, like the state, shouldn't be in the business of policing parents. Banning physical punishment might make for more effective parenting, the argument goes, but so would forcing parents to read to their children every night. Ultimately, it's up to parents to determine how to raise their children.

That's a stance that makes sense for many aspects of child-raising — but one that we don't take when it comes to matters of basic well-being and abuse. Over the 20th century, public opinion shifted as we came to accept that the state had a responsibility to prevent parents from beating their children.

Until now, we've made a strange distinction: recognizing this responsibility for punishment that leaves broken bones, bruises, or other physical signs of pain, but permitting less severe forms of physical punishment, like spanking, that do not. New research, however, is telling us that these milder forms of physical punishment still make an indelible mark on a child's brain, increasing the chance of depression, addiction, and other mental disorders down the road.

The discussion here shouldn't be about parents' rights, but children's. Physical punishment is never okay. Whether the mark it leaves is on the skin or inside the brain is unimportant.