They mention only controlling for health reasons. How do they control for that? Is it even realistic to control all the health reasons? And what about thousand

They mention only controlling for health reasons. How do they control for that? Is it even realistic to control all the health reasons? And what about thousands of other things, such as 1) People who like to take high risks, might be less keen on communal activities, like volunteering. 2) People who have bad habits might also be less likely to volunteer. 3) Depressed, stressed out, cynical people may be less likely to volunteer. 4) Poor people are less likely to donate and also have lower life expectancy. 5) People who died earlier didn't make it into that point in life where they would be able to donate. This one is easier to control though. And probably countless of other things? I'd need to go deep into the actual study to understand how they controlled for all of that, but right now it makes me very skeptical.