Smoking as "independent" risk factor for suicide: illustration of an artifact from observational epidemiology?

Two widely used criteria for determining whether an association between a risk factor and a disease is causal are dose response and independence from other factors. Data from a large US risk factor study (MRFIT) throw up a relation between cigarette smoking and suicide that meets these criteria, yet appears to be biologically implausible. It is likely that many more such associations, for other exposures and other diseases, are equally spurious, but are protected by their lack of obvious implausibility.