I have this scenario playing out within my own house. My wife and youngest are exactly like this. They eat when they are hungry and they stop when they've had enough - even if they are only half way through a plate of their favorite food. They have no problem sliding perfectly good uneaten food into the trash can. They get "full" from whatever they eat - be it healthy LC food or the junkiest sort of sugar sweetened processed food. My youngest prefers to "eat healthy" like her father. She is just as happy to eat good food as she is the tasty junk food. My wife, on the other hand, prefers eating junk along with any other healthy food that I cook. She loves her sweets. While my wife is not exactly skinny or healthy, she is within her normal BMI.

On the other had, my oldest daughter is exactly like me. Eating carbs we are ready to eat at all times. We have weak satiety signals eating good food and none at all eating carby junk. We were both heavy at an early age. We both developed binge tendencies as the backlash to resisting our near consent desire to eat. I could tell from an early age that she was exactly like me. My middle child is somewhere in the middle. She is heavy and eats too much junk food. However she can feel full, stop eating, and toss the rest of the food on her plate. My older daughter and I are "finish all the food on our plate" kind of people and if you don't want yours we'll finish that too.

I was the fat kid in class in 1969 -- back when there was only 1 or 2 of us in an elementary school classroom. That was before the food pyramid. That was before they took the fat out of everything and replaced it with sugar. It was even before they messed with the wheat to get larger yields. I got fat on normal, 1950's style American food. So yes, I believe that I am genetically predisposed to gain weight on a diet where carbohydrates contribute a significant percentage of the daily calories on a year-round basis. It is just that I got fat on bread and potatoes. When our food environment shifted towards less fat and more sugar, that just pulled more people into my category of those who struggle not to overeat.

My family is exposed the same food environment. When we all ate the SAD, two of us struggled with rapid weight gain and a near constant desire to eat. Two of us didn't have a problem with overeating at all. One of us started off OK - relatively thin, but gradually put on excess weight over time. When we all eat low carb the disparity is less, but my oldest daughter and me can still eat too much if we don't put on the breaks. We like to eat and we don't have strong satiety signals. We can do well if we watch our diets, but it is still not automatic.