Few as an example:

1. Individuals who do high risk activities might be less likely to do volunteer work.

2. Individuals who simply have bad habits in life might be less likely to volunteer.

3. People who die earlier won't even make it far enough to be in position to donate.

4. Poor people are less likely to be able to donate and can get less care for medicine, have worse conditions generally.

I see thousands of such examples, I don't think it's possible to control for all of that, is it? You would have to

1) List all of those things that should be controlled. I imagine already this to be an amazing feat.

2) Somehow gather all this data from people.