This second finding "suggests that when parents -- in this case fathers -- hit children for disciplinary purposes, it has long-term effects on children's receptive verbal capacity," Taylor said.
"This, of course, has implications for children's academic performance and general success in life," she said.
Researchers have a much better idea of why spanking influences aggression than why it influences learning ability, MacKenzie said.
Families who spank may be less likely to read to their kids or guide their language development. The stress children feel as a result of spanking also might play a part. "We know that kids who are physically abused have cognitive development problems," he said.
By assessing aggression and vocabulary at age 3, the study also tested the argument that some kids are just poorly behaved and therefore receive more spankings. They found that the delayed behavioral and cognitive effects of frequent spankings at age 5 by mothers and fathers remained firm regardless of how poor the child's early behavior was.
"It doesn't wash out the effect," MacKenzie said. "It's still there."
Although the research showed an apparent association between spanking and a child's behavior and learning abilities, it did not necessarily prove a cause-and-effect relationship.
For more information on child discipline, visit the Nemours Foundation.
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