Monday May 19 2008
This study cannot prove that exposure to mobiles causes behavioural problems
A “shocking” study has found that pregnant women who use mobile phones are “more likely to have children with behavioural problems”, the Daily Mail reported today. They said that using a mobile just two or three times a day can raise the risk of hyperactivity and emotional problems in the offspring. They add that problems are even more likely if the child then goes on to use a mobile before the age of seven.
This story and corresponding coverage in The Independent and The Daily Telegraph are based on a study in more than 13,000 women in Denmark. The study was cross sectional, meaning that it looked at a group of people at a certain point in time and comparing the characteristics of the subjects. As such, it cannot prove that one factor causes another, in this case exposure to mobiles causes behavioural problems. The researchers say their results must be interpreted with caution as other “unmeasured” factors could be responsible for their findings.
The researchers also found that children who had the most exposure to mobiles were also inclined to be from a lower socioeconomic class, to have had mothers who smoked, and to have mothers who themselves had a history of mental or psychiatric disorders. They themselves say that it is possible that “lack of attention given to a child by mothers who are frequent users of [mobiles]” may be a reason for the observed association.
On the basis of this study, headlines such as The Independent’s “Warning: Using a mobile phone while pregnant can seriously damage your baby” and the Daily Mail’s “Mobile phone menace to the unborn child” are too strong a message to be sending to the public about this issue.
Where did the story come from?
Hozefa Divan and colleagues from the University of California and the University of Aarhus in Denmark carried out the research. The study was funded by the Lundbeck Foundation, the Danish Medical Research Council and the University of California. The study was published in the (peer-reviewed) medical journal: Epidemiology.
What kind of scientific study was this?
This cross-sectional study used data and participants from an earlier study, the Danish National Birth Cohort (DNBC). The DNBC enrolled 101,032 pregnant women between 1996 and 2002 with the intention of following them for decades and gaining a “life course perspective”. The women were questioned twice by telephone during their pregnancy and twice after – when their child was six and 18 months of age. The interviews included detailed questions on lifestyle, diet, and environmental exposures.
In this particularly study, the researchers sent a questionnaire to women whose children had been born between 1997 and 1999 (i.e. they were now seven years old). The questionnaire asked about levels of exposure to mobiles. Mothers were asked how many times a day they used a mobile, how long they spent on the phone and its location (in handbag or pocket) and whether their children used mobiles or other wireless phones.
Other data were also collected about lifestyle and family history of illness (including behavioural disorders). The Strength and Difficulties Questionnaire (SDQ) assessed the children’s behavioural problems. The participants were asked to complete the questionnaire online. Those who did not respond were sent a paper version through the post. Sixty-five per cent of the eligible mothers returned their questionnaires providing data for 13,159 children.
Based on the mother’s responses to the SDQ, children were classified as “abnormal”, “borderline”, or “normal” for overall behavioural problems. Specific problems such as emotional, conduct, hyperactivity or problems with peer relationships were also assessed separately. The researchers then determined whether mobile phone use was associated with the SDQ behavioural classification. The researchers took into account other factors that may have affected the child’s behaviour such as the mother’s age, psychiatric history, smoking and socioeconomic status.
What were the results of the study?
The researchers found that children who were exposed to mobile phones both before and after birth were 1.8 times more likely to have questionnaire results indicating they had behavioural problems.
When the researchers looked at behaviour in children who had only been exposed to mobiles before birth, they found they were 1.54 times more likely to have behavioural problems. When they considered children who had only been exposed to mobiles after birth, they found them to be 1.18 times more likely to have behavioural problems.
What interpretations did the researchers draw from these results?
The researchers concluded that exposure to mobiles both before birth and after birth (though to a lesser degree after birth), are associated with behavioural difficulties around the age of seven.
The researchers say that “these associations may be non-causal and may be due to unmeasured confounding”. In other words, other factors that the researchers didn’t take into account could be distorting or masking the true relationship between behavioural difficulties and mobile exposure.
What does the NHS Knowledge Service make of this study?
The question of whether or not mobiles are responsible for behavioural problems in children is not answered by this study. More research in prospective studies is needed to ascertain this. For now, pregnant women should not be overly alarmed. This study doesn’t offer convincing evidence that there is a link between exposure while in the womb or after and neurological performance in children. Using a mobile when driving poses a bigger risk to health, pregnant or not.
Sir Muir Gray adds...
A single study us almost always too unreliable to justify action; let’s wait and see what other researchers say.