Researchers at the University of North Carolina at Chapel Hill said punishments considered physically abusive included: beating, burning, kicking, hitting with an object somewhere other than the buttocks, or shaking a child less than 2 years old.

Some surveys show evidence of a modest decline in spanking over the last 30 years, but recent surveys show that up to 90 percent of children between the ages of 3 and 5 years are spanked by their parents at least occasionally.

Zolotor and co-authors conducted an anonymous telephone survey of 1,435 mothers in North Carolina and South Carolina in 2002.

The study, published online ahead of print in the September American Journal of Preventive Medicine, said 45 percent of the mothers reported they or their partners had spanked their children in the previous 12 months, 25 percent reported spanking with an object on the buttocks and 4 percent reported using harsher forms of punishment.

"This study demonstrated that parents who report spanking children with an object and parents who frequently spank children are much more likely to report other harsh punishment acts consistent with physical abuse," Zolotor said in a statement.