Jessica comes home from middle school one day and is upset. Her Mom asks her what is wrong and Jessica responds, "All the kids at school make fun of me and say that I am chunky and big-boned. I just want to be thin like the rest of the girls!" Jessica's Mom retorts with, "Honey, don't worry what the other kids say. You don't have to be very thin like some of those other girls, just make sure you are healthy. Besides, we are all genetically preprogrammed to carry around a certain amount of body weight. That is just how we were made." Jessica's Mom is clearly referring to the _____