Sept. 24, 2009 -- Parents who spare the rod just might end up with smarter kids.
Two new studies suggest that children who are spanked have lower IQs than children who aren't, regardless of where they live.
In one study, researchers analyzed the intelligence scores of roughly 1,500 children in the U.S. who took part in the National Longitudinal Survey of Youth. They found that these scores were slightly lower among children whose mothers reported using spanking as a form of discipline.
In the other study, national average IQ scores were found to be lower in countries where spanking is common.
The research was led by University of New Hampshire sociologist Murray A. Straus, PhD, who has studied the impact of corporal punishment on child development for decades. He is a vocal opponent of the practice.
Straus was scheduled to present his findings Friday in San Diego at the 14th International Conference on Violence, Abuse and Trauma.
"The best kept secret of American child psychology is that kids who are not spanked are the best behaved and do the best in life," he tells WebMD. "You won't find that in a single child development textbook, but it is true."
Spanking and IQ
In the U.S. investigation, Straus and colleague Mallie J. Paschall, PhD, of the Pacific Institute for Research and Evaluation analyzed data from 806 children who were 2 to 4 years old at enrollment and 704 children between the ages of 5 and 9.
The children were tested for intelligence when they entered the trials and again four years later.
Even after accounting for factors that could influence IQ scores, such as parental education and socioeconomic status, spanking appeared to have a negative impact on intelligence.
The IQs of the younger children who were spanked were 5 points lower on average four years later than those of children of the same age who were not spanked. Scores among the older children were an average of 2.8 points lower among spanked children than children who were not spanked.
Straus characterized the impact of spanking on intelligence in the study as small but significant.
"Many things influence a child's IQ," he says. "This is just one of them, but it is one that parents can do something about."
In the second study, Straus analyzed data from more than 17,000 university students in 32 countries who were polled about their parents' use of corporal punishment. The answers were then compared to national average IQ scores.
Straus says IQ scores were lower in countries in which spanking was more prevalent, with the strongest association seen when children were spanked from childhood through their teens.
Critics Say Evidence Is Weak
While numerous studies have linked corporal punishment to aggressive behavior, far fewer have examined the impact of spanking on intelligence.
But earlier this month, Duke University research scientist Lisa J. Berlin, PhD, and colleagues also linked early spanking to reduced intelligence in one of the most rigorously designed studies to ever address the issue.
The researchers questioned 2,500 racially diverse, low-income moms about their use of spanking as a discipline tool for their toddlers.
They found that children who were spanked at age 1 were more aggressive than those who weren't by age 2 and they scored lower on tests to assess mental development at age 3.
"The research as a whole really paints a picture of the detrimental long-term effects of physical punishment," Berlin tells WebMD. "The message to parents is find other ways to discipline your children."
A 2002 analysis of 88 spanking studies spanning six decades linked spanking to 10 negative behaviors including aggression, anti-social behavior, and mental health issues.
More than 90% of the studies found spanking to be detrimental, says developmental psychologist Elizabeth Gershoff, PhD, who conducted the analysis.
"Parents spank to decrease bad behavior in the short and long term and to promote positive behavior," she tells WebMD. "What the research tells us is that spanking doesn't seem to be doing either of these things."
But critics say that research is highly suspect because it has largely been conducted by investigators like Straus, Berlin, and Gershoff who strongly oppose the practice.
In addition, the studies are often criticized for lacking scientific rigor -- a charge Gershoff acknowledges is hard to counter.
"We can't very well do experiments in which we tell some parents to spank their children and others not too," she says.
Straus likens the criticism to that leveled at the early studies linking smoking to lung cancer.
"For years the tobacco industry was able to destroy the studies one by one because they all had problems," he says. "No single study was truly definitive. But in the end the Surgeon General concluded that the evidence as a whole just couldn't be denied."