One of the most vocal opponents of spanking bans, Larzelere said non-abusive spanking is more effective on 2- to 6-year-olds than a dozen other tactics, including reasoning, verbal threats, privilege removal, ignoring, bribes, restraint and diversionary tactics.
"When you look at all the studies, conditional spanking led to less disobedience and less aggression," he said.
The only other method as effective as conditional spanking is putting a child in a "4 by 5-foot empty closet" for a minute, he said. "A boring, safe room."
He said spanking should be used to "back-up" time-out when a child is defiant. Milder methods should always be considered first.
"Of course, if parents use spanking too severely or as the primary method, the outcomes are more fighting and more problems," said Larzelere. "Parents need to learn all kinds of tactics that are most effective so they don't have to get to the point where they use spanking. But it does back up all other tactics."
Larzelere agrees with Holden that parents always try a positive method first, but "sometimes kids need negative consequences."
Murray Straus, professor of sociology and co-director of the Family Research Library at University of New Hampshire, agrees that spanking works. But the point, he said, is that so do non-violent methods, without the so-called side effects.
And spanking has a big "short-term failure rate," he said. Children, particularly toddlers, will repeat behaviors regardless of punishment and must do so to learn.
"With a 2-year-old nothing works," said Straus. "It takes hundreds of corrections over and over, and you have to be consistent and persistent. ... But it all works in the long run.
"The best established studies all show that without exception, [spanking] increases aggressiveness," he said. "They are more likely to hit other kids and also more likely to hit their parents."
When parents spank, they don't do it typically the first time, they do it when other things don't work and that sets a bad example.
"A normal kid -- not a bully or mean kid -- when someone is squirting water at him and they don't stop after repeatedly asking, will follow the same script," said Straus.
Surprisingly, said Straus, parents -- even college educated ones -- don't get that message because child development books devote so little space to the topic of spanking.
"I have surveyed books from the 1980s, 1990s and 2000s, and despite all the studies, they get a half a page in one of these giant books," he said. "Not a single one says, 'Don't spank.'
"The belief in American society is hard to shake. But parents believe in their heart of hearts that spanking works when others things don't."
He said he was impressed with Holden's research, which might help experts understand why spanking continues, "especially in this era."
As for Holden, he said his inadvertent study "shows what really goes on in homes."
"Every single mainstream parenting program teaches parents how to raise kids without physical punishment," he said. "There are plenty of places to turn to get help in the challenging task of childrearing. We have three and I survived."