While the women viewed the pictures, the functional M.R.I. machine monitored their brain activity. The resulting readouts revealed that overweight women’s brains were put off by exercise. Shown images of people being active, these women developed little activation in the putamen region of the brain, suggesting that they did not enjoy what they were seeing. At the same time, a portion of the brain related to dealing with negative emotions lit up far more when they viewed images of moving than of sitting. Emotionally, the brain scans suggested, they anticipated disliking physical activity much more than they expected to disdain sitting.The same approach was used to show that "many overweight people’s brains operate differently than the brains of thinner people when they look at images related to eating. In previous neurological studies, when heavier volunteers viewed pictures of food or food preparation, they typically developed increased activity in portions of the brain involved in reward processing, or an urge to like things, including in an area called the putamen. At the same time, their brains showed relatively blunted activity in areas that are thought to induce satiety, or the ability to know when you are full. These changes generally are reversed in the brains of thinner people shown the same images."
Leaner women’s brain activity, by and large, was the opposite, with the putamen lighting up when they watched others work out and envisaged doing the same themselves.
In other words, if you're a young man attracted to a woman whose eyes light up at the sight of cake, and who moans at the thought of exercise, the chances are that she's a future fatty in the making. Throw in an overstuffed mother, and you've pretty much got a guarantee that she's going to chub out in the next ten years.
Now, that's fine if you're a portly gentleman yourself, or if you happen to prefer big beautiful women. But regardless, it's a useful science-based metric for predicting if a young woman is going to blow up or not.