As you have already pointed out, people who eat nuts also engage in other healthy activities. It sort of reminds me of the studies on moderate drinkers and death. Perhaps people who are able to control their drinking after having one or two beers, have more self-control in other areas of their life, compared to those who are heavy drinkers or teetotalers who avoid it like the plague.

Even after controlling for all of this, I wonder if their is an optimal nut intake.