There's a long-held diet trick that says to eat less, you should put your food on smaller plates: The theory goes that it'll trick your brain into thinking you're eating more.

However, a study shows that the trick won't work when you need it most: when you're hungry.

While the theory is based on a scientific principle of perception known as the Delboeuf Illusion, researchers at Ben-Gurion University of the Negev have proven it doesn't work when it comes to dieting.

"Plate size doesn’t matter as much as we think it does," says Dr. Tzvi Ganel, head of the Laboratory for Visual Perception and Action in the university’s psychology department, in a release. "Even if you’re hungry and haven’t eaten, or are trying to cut back on portions, a serving looks similar whether it fills a smaller plate or is surrounded by empty space on a larger one."

The scientists discovered that study participants who had eaten recently couldn't tell if the portion sizes were different, but hungry subjects could spot if they were being short-changed.

The researchers suspect this is some hard-wired evolutionary science going on: Since we need food to live, running low on grub gives us the ability to better analyze just how much food is in front of us.

Because of this, the researchers say, the "small plate trick" can actually backfire on those watching their weight.

"When people are hungry, especially when dieting, they are less likely to be fooled by the plate size, more likely to realize they are eating less and more prone to overeating later," Dr. Ganel noted.