Variations in gender ratios support the connection between smoking and Parkinson's disease

Smokers have a decreased risk of Parkinson's disease (PD), but this association could simply be the result of a common factor causing both PD and aversion to smoking. Smoking behavior in industrialized nations has changed dramatically over the second half of the 20th century, with diverging patterns in male and female smoking rates. We therefore examined whether PD incidence changed concomitantly, as would be expected if smoking truly reduced PD risk. We performed a PUBMED search to identify relevant studies reporting male and female age-specific prevalence or incidence of PD in different countries. For each country and birth cohort we then estimated the male to female ratio in PD incidence, and correlated these ratios with the corresponding male to female ratios in smoking behavior obtained from national statistics. We show that the relative frequency of PD among women declines when the proportion of women smoking increases. The strength of this correlation (r = 0.28; P = 0.0002) is comparable to the opposite trend observed for lung cancer, and supports an overall 74% reduction in risk of PD among smokers. Our results are consistent with the hypothesis that smoking reduces the risk of PD.