The study out of Duke University looked at 2,500 white, and black children from low-income families. Low-income familes were chosen because studies show they are more likely to spank children than high-income families.
It found that children who are spanked when they are 1 are more likely to be aggressive at age 2, and perform worse on cognitive tests at age 3.
"Age 1 is a key time for establishing the quality of the parenting and the relationship between parent and the child," said study author Lisa J. Berlin, a research scientist at the Center for Child and Family Policy at .
The average number of spankings for 1-year-olds was 2.6 per week, while the average for 2-year-olds was nearly three.
Being spanked at age 2, however, did not predict more aggressive behaviors at age 3, possibly because the spanking had begun at age 1 and by age 2 the kids were already more aggressive, Berlin said.
Researchers also looked at the effects of verbal punishment, defined as yelling, scolding or making derogatory comments. Verbal punishment was not associated with negative effects if the mother was otherwise attentive, loving and supportive.
Researchers found that black children were spanked and verbally punished the most, possibly because of cultural beliefs about the importance of respecting elders and in the value of physical discipline, or because parents feel they have to prepare their children for a racist and potentially dangerous world, the researchers said.
Although the negative effects of spanking were "modest," the study adds to growing research that claims spanking isn't good for children.
"It's a parenting practice that has been around for a long time, and that's also in transition," Berlin said. "In general, the use of spanking is going down. But there is also a contingent of people who really believe in it, who say that's how they were raised and it's a tradition they want to continue."
The study is published in the September/October issue of Child Development.