The relationship between spanking and intelligence is found in children around the world, said the lead author of the study, University of New Hampshire professor Murray Straus. Children in the United States who were spanked had lower IQs -- by 2.8 points to 5 points -- than those who were not spanked, Straus found. Straus studied 806 children ages 2 to 4 and 704 children ages 5 to 9. Both groups were retested four years later. How often parents spanked influenced IQ score. 'The more spanking, the slower the development of the child's mental ability,' Straus said in a news release. 'But even small amounts of spanking made a difference.' Straus and his colleagues looked at corporal punishment practices in 32 countries by surveying 17,404 university students. The analysis found a lower average IQ in nations in which spanking was more prevalent. The strongest link between corporal punishment and IQ was for those children whose parents continued to use corporal punishment even when they were teenagers. 'It is ... time for the United States to begin making the advantages of not spanking a public health and child welfare focus, and eventually enact federal no-spanking legislation,' he said. How does spanking impact intelligence? Straus suggests that the chronic stress created by regular spanking creates post-traumatic stress symptoms in children. PTSD is linked to lower IQ. Economic status also underlies both spanking practices and IQ. Straus is a leading researcher on corporal punishment. His studies were funded, in part, by the National Institute of Mental Health. Another study, reported earlier in September, found that many low-income children are spanked as young as 1 year old, and that this practice is tied to more aggressive behavior by age 2 and delayed social-emotional development by age 3. 