WASHINGTON (Reuters) - Children who are never spanked, or hardly ever spanked, fare better on some intelligence tests than children who are frequently smacked, researchers say. It could be because parents who do not spank their children spend more time talking to them and reasoning with them, the researchers said. ``Some parents think this is a waste of time, but research shows that such verbal parent-child interactions enhance the child's cognitive ability,'' Murray Straus of the University of New Hampshire, who worked on the study, said in a statement. His team studied more than 900 children who were aged 1 to 4 at the start of the trial in 1986. They were given tests of cognitive ability -- which is the ability to learn and to recognize things -- in 1986 and again in 1990. They then accounted for factors such as whether the father lived with the family, how many children there were in the family, how much time the mother spent with the child, ethnic group, birth weight, age and gender. They watched mothers with their children and questioned them about corporal punishment. The more the children were spanked or otherwise physically punished, the lower their scores on the test, they told the World Congress of Sociology in Montreal over the weekend. ``The cognitive ability of the children who were not spanked in either of the two sample weeks increased, and the cognitive ability of children who were frequently spanked decreased,'' Straus said. He said it was not a case of the spanked child losing ability, but rather not gaining it as quickly as he or she should. ``The children who were spanked didn't get dumber,'' Straus said. ``What the study showed is that spanking is associated with falling behind the average rate of cognitive development, not an absolute decrease in cognitive ability.'' Straus said it seemed that parents who did not hit their children reasoned more with them to control their behavior. ``We found that the less corporal punishment mothers in this sample used, the more cognitive stimulation they provided to the child,'' Straus said. Straus said there was a trend against slapping and spanking children in the United States, but studies show most parents still do hit their children. He thinks there should be an education campaign. ``If parents knew the risk they were exposing their children to when they spank, I am convinced millions would stop,'' Straus said.