Differences in longevity between vegetarians and meat-eaters in epidemiological studies are often confounded by socioeconomic status, health consciousness, exercise habits, and smoking rates.

“I don't dispute for one moment that every time you do an epidemiologic survey and you compare people who live on hot dogs and pepperoni to vegetarians the epidemiology will always tell you that the vegetarians are going to live longer um and while that might be an extreme example you do appreciate that on average those vegetarians have a much higher socioeconomic status they are much more health conscious they are exercising much more they're much less likely to be smoking doing yoga all these other things and therefore how can we disentangle the variable from the effect”