Nguyen USDT, et al. - Smoking is associated with an increased risk of psoriatic arthritis (PsA) in the general population, but not among patients with psoriasis. Researchers intended to shed light on the possible methodological mechanisms behind this paradox. They found that smoking was positively related to PsA risk in the general population, but negatively associated with PsA risk among patients with psoriasis, in a large cohort representative of the UK general population. Furthermore, they noted that conditioning on a causal intermediate variable (psoriasis) may even reverse the link between smoking and PsA, potentially explaining the smoking paradox for the risk of PsA among patients with psoriasis.