Many people mistakenly state that humans have instincts. Human beings do not have instincts; only animals have instincts. Instincts are complex genetically encoded behaviors like nest-building. Humans have drives such as the drive to eat, have sex, seek physical comfort, companionship, etc.
When I mentioned this distinction to someone once after he asserted our human "instinct", he said that it was basically a semantic difference, but that is not true. The difference between a drive and an instinct is that a healthy animal can't resist it's instincts and that it is behavior it did not learn. A drive can be resisted, and we learn how to deal with our drives from our respective cultures. We see this all the time in people who choose to be celibate, starve themselves to death, or isolate themselves from others. We see these activities as difficult and often abnormal because resisting a drive is not only hard, but generally antithetical to the continuation of the species.
The reason I'm bringing up the idea of instincts and drives is that there is another distinction between them which relates to the idea that people who are fat are stupid. That is the fact that the relationship we have with our drives is an emotional one, and the one that animals have with instincts is not. Animals don't think about enjoying copulation. They merely do it because they are driven to do so. Humans attach a wide variety of emotions to how they act (or don't act) on their drives.
Eating is one of the stronger drives because it is required that we do it in order to survive. Sex, on the other hand, can be resisted without the risk of death on the part of the individual who is abstaining. That means we face our emotional relationship with eating much more often and form a more complex relationship with it than other drives.
Many people who are not fat conclude that fat people are too stupid to exercise proper diligence over their eating habits or exercise, but eating isn't something which is governed by intellect. It is emotional. Just as a woman generally cannot make an intellectual decision to be sexually attracted to a man merely because his relationship resume is perfect for her, a fat person cannot decide to not eat based on her weight. Sex and food are both drives we act on emotionally, rather than with our intellects.
There has been some evidence that links lower intelligence with higher weights, but saying being fat makes you stupid is a conclusion based on a correlation and causation error. More poor people are fat, but they are also less educated. Being fat doesn't make you stupid. Poverty is the common cause of both lower test scores and obesity. There may even be some other factors such as poor nutrition during ones formative years causing impaired intellectual development, but there is no evidence currently to support that.
The myth that fat people are stupid is just another way of attempting to justify fat bashing. The correlation between weight and intelligence is being manipulated to make it look as though weight links directly to intelligence, when it simply does not. Those who assert otherwise have an agenda which has nothing to do with concern about the negative effects of being overweight.