Carolyn, I am with you on the quantity issue. I have a very close friend who is quite thin and has always been thin. For the record her father is very thin and her mother is slightly overweight -- I haven't seen her brother since we were kids but I am guessing he's a normal size. She and I talk about emotional eating a lot, because both of us love sweets and desserts and both of us eat sweets and desserts when we are stressed or upset. However, here is the thing about her eating habits compared to mine. If I am upset about something and indulge in junk food, I will still go ahead and eat all the rest of my meals as if that indulgence never happened. My friend will eat significantly less at her subsequent meals, in addition to usually eating far less of the junk food than I do (or than I used to).
So yes, she does do emotional eating. But if we both ate an ice cream cone at 4:30 in the afternoon, I would still eat a normal dinner at 6:30 whereas she would probably have a piece of toast at 8pm and call it "dinner." I've seen her indulge at meals and eat large quantities of food (still far less than the quantities I eat even at my current low weight), but she will then not eat much at all for the next day.
Is she thinner than me because of genetics? Maybe there is some component there, sure. But really, she eats less than I do. I find that most people I know, who have never been overweight, if I look carefully at their eating I find that they eat smaller portions than I do. Does genetics make them less hungry? Does genetics make me keep eating even when I am full?
If there really is a huge biological factor, then maybe it is like having a family history of high blood pressure. If you are careful about eating and exercise, you may be able to avoid ever getting high blood pressure. There are steps you can take to reduce your risk.
I have a really hard time believing there is little to no cultural influence. Consider just the diets of different ethnic groups (including the popular "mediterranean" diet). I'm Jewish, and my comfort foods are things like dense noodle puddings, pot roasts, egg breads, etc. A lot of Jewish foods are similar to eastern european foods, which have a lot of meat and fat. OTOH, consider the Japanese diet, which is full of lean fish, vegetables, and rice. Which has more calories, a fish cake or a matzoh ball?
I know the book has a study about twins separated at birth, but I wonder if there is a study about children adopted into different countries. I know a lot of my neighbors have children they've adopted from China, who are being raised with a traditional American diet. I wonder how they will compare to a child of european descent being raised in China, in terms of their weight as adults?