I saw this study when I was in college (might not be the same one).
two things... first these studies offer an incentive such as a cash reward. "smart" fat guys have jobs and families and don't have time to take part in this, thus they are left with individuals who are looking for money.
second, individuals with lower mental abilities tend to be over weight or have poor health.
so mixing these two together, you would get "fat people are not as smart"
I guess its just like saying if you own a chevy, you are more likely to live in a trailer park
