Sometimes, you just can't say no to another spoonful of ice cream. And maybe one more after that. Why not? You deserve it. And since the tub is almost empty anyway, why not just finish the whole thing?

There's no need to beat yourself up about it later — although you probably will. The truth is you were probably doomed from that very first spoonful.

That's because there may be a switch in the brain, and for whatever reason, it can get stuck in the "on" position.

In a paper published this week in the journal Nature, researchers suggest that impulse control may come down to a very specific circuit in the brain. And that circuit always says yes.

"We discovered the brain connections that keep impulsivity in check," Scott Kanoski, a neuroscientist at the University of Southern California explains in a news release. "The key to this system is a neuropeptide that we've been focusing on, melanin-concentrating hormone, in studies on appetite and eating."

The upshot? There's a physical mechanism for impulsiveness — and understanding how it works could lead to major breakthroughs in the fight against everything from obesity to gambling to drug abuse.

That mechanism is actually a chemical produced by specialized cells in the cone-shaped area of the brain known as the hypothalamus. That small but critical region of the brain is responsible for everything from controlling body temperature to gauging our levels of thirst and hunger. It occasionally produces melanin-concentrating hormone, or MCH — a chemical linked with our desire for food or drugs. MCH production, however, appears to be independent of our actual need for food, drink or drugs. That's where, the researchers suggest, the "just-one-more" phenomenon begins.

Now, how to control it.

"By manipulating this circuit it is possible that one day we might be able to develop therapeutics for overeating that help people stick to a diet without reducing normal appetite or making delicious foods like donuts less delicious," adds lead author Emily Noble of the University of Georgia in Athens.

A medically accurate illustration of the hypothalamus It may not have a lot of real estate in the brain. but the hypothalamus carries a lot of weight on our decisions because of the hormones it releases. (Photo: Sebastian Kaulitzki/Shutterstock)

For their research, the scientists treated rats to a self-serve buffet. But the treats were on a timed delivery system, making a tasty morsel every 20 seconds, and only when a rat pressed a lever. Hit that lever too early — as impatient test subjects occasionally did — and the counter would start from scratch. Lever-happy rats would have to wait another cycle before the food became available again.

A second experiment offered the rats two dining options. Push Lever A and get an immediate small reward. Pushing Lever B meant waiting for around 40 seconds, but the food reward would be much bigger.

Guess which lever those impatient rats were most furiously fond of? That's right. They picked the now-now-now switch.

"They don't just sit there and wait," Kanoski notes. "They worked harder to achieve the same, or even fewer, number of pellets."

Their impulsiveness, however, really soared when scientists gave the rats an MCH boost.

"We would drive the system up, and then we would see the animals be more impulsive," Kanoski said. "And if we reduced function we thought they would be less impulsive, but instead we found that they were more so. Either way, they had elevated impulsivity."

Through brain scans, the team zeroed in on a neural pathway that conducted MCH from the hypothalamus to the ventral hippocampus, the part of the brain where self-control likely resides.

MCH, it seemed, could talk that old stalwart into loosening its inhibitions. The result? More please.

A brain holding a knife and fork over an empty plate. There's a part of the brain that always wants another serving, even if we don't need it. (Photo: Fabio Berti/Shutterstock)

Researchers can now see where that conversation between the brain's reward system and its impulse control center takes place. The next step will be to map it — and potentially influence the discussion. It may ultimately be possible to throttle down a food-eating frenzy. Or, as the researchers suggest, counteract the impulsive behaviors often seen with psychiatric diseases like schizophrenia and depression, and even Parkinson's disease.

"We are not quite in a place where we can target therapeutics to specific brain regions yet," Noble says. "But I think that day will come."

There's a part of the brain that always says yes to one more
The brain's impulsivity switch could be key to fighting obesity as well addiction and disease.