Most parents tell their children to stop fighting -- they scold and punish children who hit, kick or roughhouse.
But these days some moms and dads are doing just the opposite. They're encouraging their kids to fight, taking a cue from TV shows like "The Ultimate Fighting Championship," or "UFC," where moves like the "Ground Pound" and the "Cobra Strangle" have millions of fans hooked, including kids.
In "Ultimate Fighting," competitors box, wrestle, kick and do almost anything they can to knock out their opponent or make them submit. Now gyms around the country are teaching mixed martial arts, or MMA, the sport featured on the show, to kids as young as five.
Mixed martial arts fascinates people -- mostly male people, frankly.
Ultimate Fighting Championship, the sport's biggest promoter, caters to guys who always wondered whether karate is more effective than judo, or who would win in a battle between a wrestler and a boxer. MMA answers those questions, and the success of UFC has spawned a slew of spin-offs and feature films.
And now kids want to do it. Some tape themselves fighting and post the video on YouTube, and sometimes adults even construct cages so kids can fight like the pros.
Other children fight in gyms, on wrestling mats, and like the pros sometimes they get hurt. But unlike the pros, these young fighters sometimes cry and need to be comforted by their parents, as happened at a recent tournament in Santa Ana, Calif.
Some say MMA is a terrible thing for kids to be doing. Presidential candidate Sen. John McCain called the adult version "human cockfighting" and wrote a letter to the governors of every state asking them to ban it. Many states have.
Mayor Robert Correia of Fall River, Mass., was horrified to learn that in his town, not far from his office, one very successful ultimate fighting academy called Gillett's Gym teaches MMA to young boys and girls. The mayor says the sport sends the wrong message to kids and wants the gym shut down.
"To allow this to be taught to our children and for adults to stand by and cheer this on?" he said. "It's telling them 'look, the best thing to do is hurt someone.'"
But do the parents of the kids at Gillett's think the sport will teach kids to settle disputes with fists instead of words?
"No, my kids have never left here and used it outside of this gym," one parent said. "Never."
In fact, parents often get in the ring themselves, and some said that it's a good way to interact with their kids and that it gets aggressive impulses out in a safe and fun way. There are rules against things like punching in the neck and twisting fingers.
"He likes it as a sport and he likes it to teach him right from wrong," said Mindy Poirier, of her son's MMA experience.
Kim Velozo's 13 -year-old daughter Nicole fights, and Velozo said it's about self-defense.
"If somebody jumps her or something like that, she'll know how to defend herself against whatever," she said. "She loves it. She loves it."
Many of the moms said the training even helped their kids outside the gym, with discipline and following directions.
All the moms "20/20" talked to said mixed martial arts has been good for their kids. Correia doesn't think those parents know how unsafe it is.
"I've heard from doctors who are very concerned about this going on," he said.
"I think it's dangerous from a physical standpoint," pediatrician Lisa Thornton recently told "Good Morning America." "It can lead to significant injuries to the neck and bones and ligaments."
"Mixed martial arts is coining the phrase which means, anything goes," Correia said. "They don't have any rules that are set up by any governing body."
Actually they do have a governing body, the North American Grappling Association, but so what? Hockey has a governing body, but that doesn't stop the fights, even in peewee hockey. No sport is free from injury.
In six years, 77 kids died when they were hit in the chest with a baseball. Many more died riding bikes. No one has yet gathered comparative statistics on the risk of mixed martial arts, but even cheerleading sends thousands of kids to hospitals every year.
MMA participants wear head gear and big gloves to stay protected, and the sport's governing body says it has banned the moves that are most likely to lead to injuries in children. A study from Johns Hopkins found that in MMA there are no more injuries than in boxing.
The kids at Gillett's Gym say they never worry about getting hurt. "We're not in there to hurt each other," said 8-year-old Justin Pereira. "We're all friends here. All the kids are friends and we're just in there to like, go have fun and just learn."
Justin said it helps him learn respect in all areas of his life. "It's like, you got to respect someone that's part of your family or your teacher," he said.
So do politicians really need to step in and act on behalf of parents?
"Are all parents responsible?" Correia said. "Obviously not."
So does society have a right to protect children when some parents aren't protecting them? Correia thinks so, and moved to close Gillett's without ever visiting the gym.
"I looked at the Web site," he said. "The parents do have a chance to decide that through their elected officers. That's what a democracy is all about. We run across that every day in the news, parents who are abusing their children."
But the parents "20/20" talked to don't think they're children are being abused, and neither do the kids.
"I'm his mother," Justin's mom, Connie Pereira, said. "I know what's right for him. I have no problem with him coming here. I don't care what the mayor's saying."