The single biggest risk factor for PD is advancing age. Men have a slightly higher risk compared to women. Family history is also an important risk factor. Individuals with an affected first-degree relative (parent or sibling) are estimated to have an approximately doubled risk for developing PD. This increased risk is likely to be a combination of environmental and genetic factors that close relations have on common.
The single factor that has been most consistently associated with a reduced risk of PD is cigarette smoking, which has been demonstrated in numerous studies. It is not known whether smoking confers a genuine protective effect, or whether individuals who are prone to develop PD for other reasons are also prone to avoid smoking. Nonetheless, the negative impact on general health from smoking is enormous, far in excess of any slight reduction in risk for PD. Smoking cannot be recommended as a strategy for avoiding PD.
Caffeine consumption is also associated with a reduced incidence of PD. In women, hormone replacement therapy appears to be associated with a reduced incidence in women who consume only small quantities of caffeine, but may be a risk factor in those who consume more than five cups per day.