SAN DIEGO –  Don’t spank your kids if you want them to be very intelligent. A ground-breaking research has found that children who are spanked have lower IQs.

Corporal punishment is extremely stressful and can become a chronic stressor for young children, says Murray Straus, professor at the University of New Hampshire.

“All parents want smart children. This research shows that avoiding spanking and correcting misbehavior in other ways can help that happen,” says Straus.

“It is time for psychologists to recognize the need to help parents end the use of corporal punishment and incorporate that objective into their teaching and clinical practice,” he says.

Straus found that children in the US who were spanked had lower IQs four years later than those who were not spanked.

Straus and Mallie Paschall, senior research scientist at the Pacific Institute for Research and Evaluation, studied nationally representative samples of 806 children aged two to four, and 704 kids ages five to nine. Both groups were retested four years later.

IQs of children aged two to four who were not spanked were five points higher four years later than the IQs of those who were spanked.

The IQs of children aged five to nine years old who were not spanked were 2.8 points higher four years later than the IQs of children the same age who were spanked.

Straus and colleagues in 32 nations used data on corporal punishment experienced by 17,404 university students when they were children.

“How often parents spanked made a difference. The more spanking the slower the development of the child’s mental ability. But even small amounts of spanking made a difference,” Straus says.

His analysis indicates the strongest link between corporal punishment and IQ was for those whose parents continued to use corporal punishment even when they were teenagers, says a New Hampshire release.

Straus said corporal punishment can become a chronic stressor for young children who typically experience punishment three or more times a week. For many it continues for years.

These results were presented Friday at the 14th International Conference on Violence, Abuse and Trauma in San Diego.

They have also been published in the Journal of Aggression Maltreatment & Trauma.