Smacking and children's IQ

Friday September 25 2009

“Smacking your children can damage their mental ability,” said the Daily Express . It reported on a “ground-breaking” study that tested the IQs of 806 children aged two to four and 704 children aged five to nine. Four years after first being tested, the younger children that had been smacked had IQs five points lower than those who were not smacked, and the difference was 2.8 points in the older children.

This study is based on data collected over 20 years ago, and parenting practices are likely to have changed in this period. Other limitations include the fact that use of smacking was only assessed for a two-week period, only the mother’s use of smacking and not the father’s was assessed, and the study was largely based on parental report and did not assess the severity of the smacking.

Overall, this study showed a surprisingly high rate of corporal punishment and some link between smacking and cognitive performance. However, the effect seen was relatively small and may have been linked to factors other than smacking itself.

Where did the story come from?

Professor Murray A Straus and Mallie J Paschall carried out this research. No sources of funding for the study were reported. The study was published in the peer-reviewed Journal of Aggression, Maltreatment & Trauma .

What kind of scientific study was this?

This study looked at children of women enrolled in a cohort study started in 1979, and included both cross-sectional and cohort analyses (looking at changes over time). It aimed to look at the effects of corporal punishment, such as smacking, on the cognitive abilities of children.

The researchers looked at data collected in 1986 for 806 children aged two to four years and 704 children aged between five and nine. The children’s cognitive ability was tested in 1986 then again in 1990. Different tests were used at the two time points. The children’s scores were standardised so that they indicated how far above or below the average level of cognitive ability each child was, relative to similarly aged children in the study. This method makes 100 points the average score for any group.

The mothers were asked about their parenting practices and their child’s behaviour.

Overall, 1,510 children were included in the analyses. Children who were excluded from the study for not having complete data were more likely to have a lower birth weight and have mothers who had not completed high school, and more likely to be from single parent families.

The use of corporal punishment was assessed for a week in 1986 and again in 1988. The mothers were interviewed at these times, and interviewers recorded whether mothers smacked or hit the child during the interview. Mothers were also asked if they found it necessary to smack their children in the past week, and how many times.

The researchers then combined the observation and interview reports for both weeks to classify children as one of four corporal punishment levels: those who experienced no corporal punishment, those who experienced one instance of corporal punishment, those who experienced two instances, and those who experienced three or more instances.

The researchers tested how corporal punishment (assessed in 1986 and 1988) and cognitive ability at the start of the study (1986) related to cognitive ability at the second assessment in 1990. They adjusted for birth weight, sex, age and ethnic group, mother’s age at birth, mother’s education, cognitive stimulation and emotional support by the mother, number of children at home and whether the father was living with the mother at the start of the study.

What were the results of the study?

The study found that 93.4% of two to four year olds and 58.2% of five to nine year olds were smacked at least once in the two combined week-long assessment periods.

Children who were smacked were more likely to have lower cognitive ability at the start of the study, have less maternal emotional support, to be younger and to have mothers with lower levels of education. When the researchers took all of the factors into account, they found that smacking was associated with lower cognitive ability scores among the younger and older groups of children. For each point a child increased on the four-point corporal punishment scale, they reduced by 1.3 points on the cognitive ability scale if they were in the two to four age group, and 1.1 points if they were in the two to four age group.

The decreases in score do not represent a reduction in cognitive ability, rather lower development of cognitive ability compared to average of the group.

The two to four year olds who were not smacked in either week gained an average of 5.5 cognitive ability points compared to the average, and the five to nine year olds gained an average of almost two points. The two to four year olds who were hit three or more times neither gained nor lost compared to the average, and the five to nine year olds lost an average of almost one point compared to the average.

For two to four year olds, cognitive stimulation from the mother had a greater effect on cognitive ability than corporal punishment. In children aged five to nine years, corporal punishment and cognitive stimulation from mothers had similar sized effects on cognitive ability. A child’s cognitive performance at the start of the study also had a significant effect.

What interpretations did the researchers draw from these results?

The researchers suggest that there could be a “bidirectional” relationship between corporal punishment and cognitive ability, with parents more likely to smack a cognitively “slow” child, but also that corporal punishment slows the rate of further cognitive development. They say that if the findings of this study are confirmed by other studies, programmes targeted at making clear the benefits of avoiding corporal punishment could reduce its use and lead to a “national enhancement of cognitive ability”.

What does the NHS Knowledge Service make of this study?

There are a number of points to note when interpreting this study:

  • This study only assessed the use of smacking over two week-long periods, and was based solely on the mother’s report and her behaviour in front of the interviewer. It is possible that this method missed some children who were smacked at other times, or was affected by the mothers’ inability or reluctance to recall how often the child was smacked.
  • The data in this study was collected more than 20 years ago and there are likely to have been changes in attitudes to corporal punishment over this time. This means the results may not be representative of the present situation.
  • The study did not assess the severity of the smacking or paternal use of corporal punishment, which could have an effect on outcome.
  • It is possible that these differences do not relate solely to smacking. There may be other differences between the groups of children that had been smacking and those that had not that are having an effect. This possibility is supported by the fact that children who were smacking already had lower cognitive abilities at the start of the study than those who were not.
  • Different tests of cognitive ability were used at the start and end of the study. Although both scores were standardised so that they related to average scores within the group, the use of different tests may mean that comparing the scores at the start and end of the study may not be appropriate.

Overall, this study showed a link between smacking and cognitive performance, but the effect seen was relatively small and may have been linked to factors other than the smacking itself. As the authors suggest, it needs validation by other studies.

It is surprising that a total of 93% of the mothers of children aged from two to four and 58% of mothers of children aged from five to nine used corporal punishment in the two-week test period, suggesting that these 25-year-old results might not apply to contemporary parenting practices.

Analysis by Bazian
Edited by NHS Choices