And here is Richard A.:
The Body-Mass Index of Twins Who Have Been Reared Apart
We conclude that genetic influences on body-mass index are substantial, whereas the childhood environment has little or no influence. These findings corroborate and extend the results of earlier studies of twins and adoptees. (N Engl J Med 1990; 322:1483–7.)
IOWs, the reason why white kids of today are much fatter than white kids of the 50s and 60s is due to genetic influences and environment has little or no influence
This shows that the twin studies are flawed.
Because genetic changes are far less than can explain the changes in weight.
But remember that the changes in weight are a result of small changes in behavior 10 lbs of steady state weight comes from just 150 calories extra per day. Thats 10 minutes walking. 3 oreos, one coke? Seems small too, and maybe a small drift on a single gene can achieve that end?
For the average american male every 1 point of BMI is about 3.2 kg, or about 7 pounds. Since 1970 we have probably added about 3 points to our average BMI, but maybe less for the median. So we are trying to explain 21 lbs.