In the US, the practice of spanking children isn't yet quite as dated as this photograph (Wikimedia Commons).

Past research has focused mainly on children’s shorter-term behavioral outcomes due to maternal spanking. But the most recent Columbia study also includes the effects of paternal spanking, as well as the longer-term effects of spanking on the child's aggression and vocabulary.

The study looked at the relationship between children spanked more frequently (more than twice a week) or less frequently (less than twice a week). It then evaluated the children's scores on the Child Behavior Checklist (CBCL) and the Picture Vocabulary Test (PPVT). It also looked at the overall prevalence of spanking in the US.

Children ages three and five who were spanked by their mothers frequently, i.e., more than twice a week, had significantly higher incidences of externalizing bad behavior at age 9 than children who were not spanked. Even five year olds who were spanked by their mothers less frequently, i.e., less than twice a week, displayed more behavioral problems, as dictated by the CBCL. There was also a strong correlation between mothers and fathers spanking 5 year olds and children’s low scores on the PPVT at age 9.

These outcomes were evident even when researchers controlled for other risk factors that may lead to behavior or vocabulary problems, such as socioeconomic status, maternal wellness, and the child's own temperament in infancy.

Over half of the three and five year olds in the study were spanked by their mothers. 33% of 5 year olds and 52% of 3 year olds were spanked by their fathers.

Perhaps the prevalence of spanking in the US makes it hard to believe that, in the long-run, scientific evidence points to its ineffectiveness. The US is “one of the few high-income countries that have not followed Sweden’s lead in banning spanking,” the study said.

MacKenzie recognizes that the topic elicits strong emotions from families, both in favor of and against spanking. In an opinion piece for CNN, he acknowledged that discipline is “intimately tied to cultural, religious and family traditions,” so it can be difficult for many Americans to look objectively at the evidence against spanking. The next step may be for researchers to help parents find more sensitive alternatives to spanking, said MacKenzie.

Reach Staff Reporter Jacqueline Hernandez here.