In the largest study of its kind, a team of researchers studied the medical records of over one million men in Sweden dating back over a period of 24 years and compared rates of hospital admission for attempted suicide against IQ scores. The research is published today in the 'British Medical Journal'.
Out of a cohort of 1.1 million men with IQ measured in early adulthood, almost 18,000 had been admitted to hospital at least once for attempted suicide. Even after adjusting for factors such as age and socioeconomic status, the researchers found that men with lower IQ scores were increasingly likely to have attempted suicide at least once. By far the most common method used was poisoning, for example taking an overdose of medication.
Dr David Batty, a Wellcome Trust Fellow at the Medical Research Council Social and Public Heath Sciences Unit, who led the study, says: "We have found a clear link between IQ and attempted suicide in this group of men. In common with some previous, smaller studies, we have shown that men with lower scores have a markedly greater risk of attempted suicide than men of higher IQ."
The researchers suggest a number of possible explanations that might underlie the association. Firstly, low IQ tends to correlate with lower socioeconomic status and income, and so individuals with lower IQ may experience more social and financial disadvantage, leading to an increase in suicidal thoughts and behaviours. Lower IQ has also been associated with poor health behaviours such as binge drinking, which also increases suicide risk. However, the researchers believe that these factors are unlikely to fully explain the observed associations.
IQ may also influence an individual’s ability to deal with stressful circumstances or traumatic events; studies in children and adolescents suggest that those of higher intelligence are more resilient to stress. Previous research has suggested that individuals with lower IQ scores may have poorer problem-solving abilities and, in times of crisis, be less able to identify practical solutions to their problems. Alternatively, individuals with higher verbal IQ scores may have a greater ability to talk about and share emotion or distress and this, in turn, might reduce the risk of self-harm.
Another possible explanation, though one which the researchers were unable to investigate further, was the role of violence early in life. Exposure to violence early in life, either directly as a victim or indirectly as a witness has been previously shown to influence both IQ or academic performance and future risk of suicide or suicidal thoughts.
"Suicide, either attempted or actual, is a serious problem, particularly among young adults, but we have a relatively poor understanding of what leads a person to take such drastic action," says Dr Elise Whitley. "If we can better understand the association between IQ and suicide, this will provide valuable insight into why some people make such a tragic decision. Such knowledge would help inform public health strategies and provide help and support for vulnerable groups."
The researchers caution that, because the analyses looked at hospital admissions in Swedish men aged 16 to 57, the results can't necessarily be generalised to other countries, to women or to older men.