To me, it's a matter of both genetics and environment. Some of us are programmed to store fat more quickly and more efficiently than others, but we weren't engineered either to be morbidly obese--that
is a result of environmental factors. I fail to see how a really high level of stored fat, save in rare cases such as the one of the leptin girl, could have been conceived as a natural mechanism, because it completely defies to point and tendency of evolution. If the mechanism has gone haywire, I'd bet it's because it wasn't given a limit simply because there wasn't any need for a limit (the way Earth used to be, we weren't meant to live in a surabondance of food)... and we've been pushing those limits more and more with inappropriate quantities and uality of food. Cf. some countries in which food circumstances aren't favorable, and people have a hard, hard time even trying to put on a just a few
more pounds. If we all had to live in the savannah, hunt our own food, pick our own fruits and keep on the move constantly for more food sources, we probably would have a hard time gaining on weight quickly.
To put it in terms that my colleagues at work would like, "if it's not broken, don't fix it". We've tried to fix something that wasn't broken, and now we've indeed broken it. Great. Back to the drawing board?
Okay. So there is a genetic predisposition, and I think the author put it into light well enough, but I'm really not convinced that this should ever be used as a convenient excuse. We were made to adapt, now it's time to adapt to that as well, hard as it may be. The role of environment IMHO is just as much, if not MORE important that that of biology. And I'm still sticking to my guns: the process of storing fat more easily isn't a curse per se, it has just become a curse because our world has made it so.
(There are other things I want to add, regarding the studies on twins, but I don't have the book with me now, so I'll post again after having double-checked the info.)