Let me try to explain a little better... IF being stupid makes you fat, then you have to be stupid to be fat.  You can't be fat and smart then.  It's impossible.  If you have an IQ of 150 or even 120, you can't ever get fat.  You're not stupid, so you can't get fat.  You are literally too smart to be fat.  If A is a primary cause for B, B will rarely happen without A. IF being fat makes you dumber, then the entire IQ curve is just shifted lower for people who are fat.  So you could have an IQ of 160 skinny that sinks to 150 fat.  You can have an IQ of 120 that shifts to 110 fat.  So you can be of whatever intelligence and the whole curve just shifts to the left, so you'll find fat people with IQs of 70, 85, 100, 115, 130, 145--the whole range.  Just a lot less at the high end and more at the lower end, and that moves the average down. The cause-effect relationship determines whether the IQ curve is shifted or whether only dumb people, with rare exceptions, become fat.