Let's look at some of the points raised by soccertese.

If the environment is part of the etiology of Parkinson's, one would expect people who shared the same environment as a PwP to have an increased chance of developing the disease. Blood relatives will share in part the same environment, but they will also partially share the same genetics, which will cloud the environmental effect. However, in large part, spouses of PwP will be free of this, so the increase in their risk of developing PD gives a more pure measure of the environmental effect. A study [1] based on data from Iceland shows risk ratios for relatives of all PwP/ late onset PwP:
siblings(6.3/6.7),
off-spring (3.0/3.3),
nephews and nieces (2.4/2.7),
first cousins (1.4/1.3),
spouses (1.9/1.1).

In a wider context, you would expect geographical variations in the prevalence of PD. Willis et al. demonstrate this at the US county scale [2].

I would strongly recommend that anyone with an interest in PD look at the maps in their paper.

References:

[1] "FAMILIAL AGGREGATION OF PARKINSON’S DISEASE IN ICELAND"
Sigurlaug Sveinbjorndottir et al.
NEMJ 2000
MMS: Error

[2] "Geographic and Ethnic Variation in Parkinson Disease: A Population-Based Study of US Medicare Beneficiaries"
Allison Wright Willis,a,* Bradley A. Evanoff,b Min Lian,b Susan R. Criswell,a and Brad A. Racette
Neuroepidemiology, 2010, Apr
Geographic and Ethnic Variation in Parkinson Disease: A Population-Based Study of US Medicare Beneficiaries

John