The study, involved only U.S. children. The researchers tested the kids' IQs initially and then four years later. Both groups of kids got smarter after four years. But the 2- to 4-year-olds who were spanked scored 5 points lower on the IQ test than those not spanked. For children ages 5 to 9, the spanked ones scored, on average 2.8 points lower than children who were not spanked. The results, he said, were statistically significant The IQ of children who were spanked is lower than those who were not spanked. In conclusion Straus stated "Contrary to what everyone believes, being hit by parents is a traumatic experience," Straus said. "We know from lots of research that traumatic stresses affect the brain adversely." Also, the trauma could cause kids to have more stressful responses in difficult situations, and so may not perform as well