Quote: from Atti at 7:17 pm on Oct. 11, 2008
Both.
I'd say a lot of it's genetic, but partially cultural as well.
I'm a mutt (Greek, English, Irish, and a number of other things in smaller percentages) and I can eat until I get sick and not gain an ounce, yet I have friends that are higher percentage or pure English/Scottish/Irish and hardly eat at all and have trouble keeping weight off. I grew up in an area that had a significant Jewish population, and I literally have never seen a fat Jew. 
Odd, there are at least three fat Jewish people that I know of, and one of them is plainly obese. Most do seem pretty skinny, though.
I've never seen an obese Asian. Minorly fat ones, though.
My whole mom's side of the family(English, Irish, Scottish, French) are really skinny, except for my one uncle. On my Italian dad's side, though, like 60% of the relatives are overweight..