Tuesday December 7 2010
The study didn't prove that mobile phones cause bad behaviour
“Mums-to-be run a greater risk of having a naughty child if they regularly use a mobile when pregnant,” The Sun reported. It said that doctors believe that microwave radiation emitted by the handsets could “wreak unseen damage on an unborn baby's brain, leading to behavioural problems”.
This study looked at mobile phone use by pregnant women and their children up to the age of seven. It found that regular use was associated with a greater risk of behavioural problems in the child.
This study is not robust evidence that mobile phones cause bad behaviour in children. There may be a number of other factors affecting behaviour that have not been taken into account in this research. It is also unlikely that mothers would be able to recall accurately their mobile use in pregnancy, seven years after giving birth.
Research to date appears to show that these devices are not harmful to children, but it is better to take precautions. The Department of Health advises that children should only use mobile phones for essential purposes and keep all calls short.
Where did the story come from?
The study was carried out by researchers from the University of Southern California, the University of California and the University of Aarhus, all in the USA. It was funded by the Lundbeck Foundation, the Danish Medical Research Council and the UCLA School of Public Health. The study was published in the peer-reviewed Journal of Epidemiology and Community Health.
Both The Sun and the Daily Mail covered the study accurately, though both appear to rely heavily on a press release of the research. Both papers included comments from other experts disputing the study’s findings.
What kind of research was this?
This study investigated whether mobile phone use in pregnancy and early childhood affects the risk of behavioural problems at the age of seven years. Cohort studies such as this can show associations between exposures (such as mobile phone use) and health outcomes (such as behavioural problems), but cannot prove cause and effect.
This was the second study on the subject by these researchers. The first, in a different group of almost 13,000 children, found that exposure to mobile phones in the womb and in early childhood was associated with greater incidence of behavioural difficulties. This new study investigated the same research question in a larger group of nearly 29,000 children.
The researchers say that previous studies of mobile phone use have looked at possible health effects in adults, while children are thought to be potentially the most susceptible to environmental exposure. They point out that the last decade has seen a large increase in mobile phone use and during the same period an increase in childhood behavioural problems.
What did the research involve?
The researchers used data from the Danish National Birth Cohort (DNBC), a long-term study set up to investigate how influences between conception and early childhood affect health in later life. The DNBC enrolled nearly 100,000 pregnant women between 1996 and 2002.
At the start, the women were interviewed four times by telephone, twice during pregnancy and twice within 18 months of giving birth. In the interviews they were asked about various lifestyle factors, dietary habits and environmental exposures. When the children reached seven years of age, the mothers were sent a questionnaire that focused on their child’s health. The questionnaire also asked about their and their child’s use of mobile phones, whether they themselves had used mobile phones during their pregnancy, their use of hands-free equipment and where they kept the phone (in their handbag or pocket, for example).
This questionnaire also asked about social conditions, family lifestyle and diseases in childhood, including detailed questions about behavioural problems, as defined by a standardised questionnaire. Based on this, children’s behaviour was classified as normal, borderline or abnormal.
For this study, the researchers used data from 28,745 of the children born between 1997 and 1999 and their mothers. Using standard statistical methods, they analysed the relationship between mobile phone use in pregnancy and in early childhood, and the risk of behavioural problems at the age of seven. They also looked at many possible confounders (other factors that might have influenced the results) such as gender, the psychiatric health of the parents and alcohol use, and adjusted their findings to take these into account. They report the results of the adjusted analyses.
The results from this new study were then compared with their previous study.
What were the basic results?
More than 35% of children were using a mobile phone at the age of seven, but less than 1% used it for more than an hour a week. Nearly 18% of the children had mothers who had used mobile phones during pregnancy and had used mobiles themselves. About 40% of the children had no exposure.
About 3% of the children scored abnormal on behavioural issues, while another 3% were classified as borderline.
More detailed analysis of the behavioural data showed the following.
- Children whose mothers had used a mobile phone in pregnancy were more likely to have behavioural problems (adjusted Odds Ratio [OR] 1.3, 95% CI 1.1 to 1.5) than those whose mothers had not.
- Children who themselves used mobile phones at the age of seven were more likely to have behavioural problems than those who did not (adjusted OR 1.2 95% CI 1.0 to 1.4).
- Children who used mobiles and whose mothers had used mobiles too were the most likely to have behavioural problems (adjusted OR 1.5 95% CI 1.3 to 1.7). The risk was smaller than found in the previous study (adjusted OR 1.9 95% CI 1.5 to 2.3).
- The risk varied according to the child’s birth year, decreasing between 1998 and 2001.
How did the researchers interpret the results?
The researchers say the results replicated those of the previous study and make it unlikely that the first finding was by chance, although the risk estimate for joint exposure by both mother and child was higher in the original study. They also point out that they included confounders that were not considered in the previous study, yet the association between mobile phone use in pregnancy and early childhood, and behavioural problems, remained.
It is not possible to conclude from this study that using mobile phones causes bad behaviour in children.
There may be several other factors affecting behaviour that have not been taken into account in this study. The authors suggest that it's possible that a mother's mobile phone use indicates her level of attentiveness to her child and that this may affect behaviour, not the use of the phone itself. They made attempts to take this into account in their analysis by adjusting for whether or not the mother breastfed in the first six months. They say that “if breastfeeding and time spent with child are good measures of mothers’ attention then we believe that our results do not support inattention as a likely explanation for the observed association”. This is a tenuous link, however, and there are good arguments that whether or not a mother breastfeeds and how much time she spends with her child do not necessarily correlate with how attentive she is. This is unlikely to be an adequate adjustment for attentiveness.
The study has further limitations. For example, it is unlikely that mothers would be able to recall accurately and in detail their mobile phone use during pregnancy, some seven years later. Also, the variation in risk between children of different birth years is unexplained.
It is also worth pointing out that although the increase in risk of behavioural problems sounds large, the vast majority of children had no behavioural problems, with only about 6% being considered abnormal or borderline.
Child health and environmental health experts have pointed out that it is difficult to see how mobile use could affect an unborn baby. They say that the radiofrequency radiation that mobile phones emit is highly localised to the part of the head closest to the phone and there is no evidence to suggest that other parts of the body are affected.
Research to date suggests that these devices are not harmful to children, but it is better to take precautions. A recent study by the World Health Organization found no association between children’s mobile phone use (up to 12 years) and cancers of the brain and nervous system. However, some uncertainties remain about the safety of the highest users, and research continues. The Department of Health advice is that children should only use mobile phones for essential purposes and keep all calls short.