Can’t wait to read the rest of this paper about cigarettes actually causing poor mental health in The British Journal of Psychiatry. The abstract cites the researchers’ results:

Adjustment for confounding factors revealed persistent significant . . . associations between nicotine-dependence symptoms and depressive symptoms. … [T]he best-fitting causal model was one in which nicotine dependence led to increased risk of depression. The findings suggest that the comorbidity between smoking and depression arises from two routes; the first involving common or correlated risk factors and the second a direct path in which smoking increases the risk of depression.

In other words:

1. The symptoms of nicotine addiction and depression are strongly connected, even when you control for variables.

2. After looking at a bunch of different ways of figuring out what causes what, the way that made the most sense was that nicotine addiction increases the risk of depression.