It’s long been known that there is an association between mental health problems and smoking. However, it’s not clear if there is a causal relationship between the two. For example, can nicotine dependence be a factor in the development of a mental disorder such as depression? Or do people with depression or schizophrenia end up smoking more as a way to deal with the stress of being unwell?
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A new study in the current issue of Journal of Affective Disorders indicates that smoking may lead to an increased risk for depression, but that genetics also play a role. These guys used some fancy statistics and a large sample of adult male twins to create a model where both shared genetics and nicotine dependence explained developing major depressive disorder (MDD). The model didn’t support MDD causing nicotine dependence, however.
Although it’s a good study (twin studies are always interesting), I think it could be made better by looking at adolescents, who are often experiencing their first episode of depression, instead of adults, who may be experiencing recurrent depression or have had more of a chance to change lifestyle factors (not just smoking, but diet, exercise, etc) due to the depression, or be affected by medication.
And of course only males were in this study, so there could very well be sex differences in this model, as well.