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Is the link always causal?
Suicide is more frequent among people who are unemployed.1 The suicide-unemployment association has been debated since sociologist Emil Durkheim’s classic study2 over 100 years ago concluded that unemployment increased social isolation, which then raised the risk of suicide. He further concluded that the number of suicides in a society did not have any specific association with the occurrence of mental disorders at the ecological level.
Many studies have suggested that the suicide-unemployment link is causal, or partially caused by a selection process governed by the effect of common unobserved factors, such as mental illness, leading both to unemployment and suicide, or that the link is reverse causal, so that a suicidal behaviour leads to unemployment, or more rarely argued, that there is no indication of unemployment causing suicide. The core problem is that, as Karl Pearson (a founder of modern statistics) said: only correlation and not causation can be estimated from observational data. This viewpoint, however, has recently been relaxed by, for example, Jamie Robins who introduced the concept of counterfactual and by econometricians who for more than 50 years have been using instrumental variables to pseudo-randomise individuals to exposure. Professor Judea Pearl’s new book is a brilliant introduction to these and other techniques used to strengthen causal reasoning.3
Blakely and colleagues say that the suicide-unemployment association found in their paper is likely to be causal.4 They argue that the link is not mediated by financial stress (which by the way carried surprisingly little information in the first place), as the incidence related to unemployment is comparatively unchanged in the adjusted regression, and because the odds of linking suicide were almost the same between the most socioeconomically deprived 50% of small areas compared with the least deprived 50%. This is further used as a vehicle to argue that the association is likely to be underestimated. However, ecological information in a micro data study might introduce “hierarchical” measurement error, which the authors acknowledge by suggesting this as a target for future studies.
As in other studies,5,6 Blakely and colleagues4 find that the suicide-unemployment association in part is mediated by mental illness, which they primarily conclude from their sensitivity analysis of biases. They further explain that mentally ill people would to a larger degree be non-active on the labour market rather than unemployed, which then suggest a sensitivity analysis for the group of those who are non-active on the labour market. Although Blakely and colleagues4 use a set of external information that differs from the information used in the reference by Sander Greenland, the sensitivity analysis is one of the virtues of the paper, as it demonstrates an approach to deal with missing confounder information. Their lowest estimated relative risk of suicide among the unemployed (1.35) is quite similar to the rate found in a study where information on mental illness was included.6 This study, on the other hand, includes only information from population based hospital discharge records, and finds also higher suicide rates among the mentally ill than the rate used by Blakely and colleagues.4
It might be hypothesised that the suicide-unemployment association differs among people who suffer from a mental disorder, as studies have suggested no association7 or even a non-significant 30% reduction in risk,8 and as individual longitudinal studies of deliberate self harm and unemployment do not present a coherent picture.1 One study even finds that the suicide rates increased with increasing income among patients.9 This might be the effect of an increased stigma10 or because employed patients are in a particularly stressful situation.
As acknowledged by Blakely and colleagues,4 their study does not provide strong evidence in favour of the hypothesis, but they try to mend imperfections in their data, and we should encourage studies such as this.
Is the link always causal?