Why do we always have room for dessert?

You're at a restaurant and just devoured a hearty meal. With an uncomfortably full stomach and seemingly tighter pants, you can't fathom eating another bite — that is, until the dessert tray passes by and your hunger returns. But why does the sight of sweet treats open up a mysterious empty compartment in your tummy? 

The secret is variety, according to Len Epstein, a SUNY distinguished professor of pediatrics and chief of the Division of Behavioral Medicine at Jacobs School of Medicine and Biomedical Sciences at the University at Buffalo in New York. 

"Part of the reason why people stop eating a meal is that they're tired of the food; they've eaten it [and] there's no more stimulation," Epstein said. "They know exactly what it tastes like. But if you introduce a new flavor, smell or even texture into the mix, "it's easy to overcome that feeling of 'Oh, I'm full,'" Epstein told Live Science. 

Related: Why do our stomachs growl?

This phenomenon is called sensory-specific satiety, which a person can experience when their interest in a certain food declines because they have eaten it repeatedly, while a novel food item may become more appealing. For example, in a 2011 experiment led by Epstein, 32 women were assigned to receive a macaroni-and-cheese meal either five times in one week, or once a week over five weeks. The researchers found that women who were presented with mac and cheese daily ate less of it than women who were given it once a week. 

In a separate study published in 2013, however, Epstein split 31 children into three groups: one that received the same mac and cheese for five days, another group that was given different brands of mac and cheese, and a final group that was provided with a variety of energy-dense foods, such as chicken nuggets and cheeseburgers. The children with the variety of options consumed much more than the kids who were offered only mac and cheese. This pattern is the same when people are exposed to sweet desserts after a salty meal, according to Epstein, or if you were eating a buffet with a wide variety of dishes. 

"You can keep presenting new foods and have people keep eating until the point where they just can't eat anymore," he said. "But that is one of the reasons for why people eat more than they need to." 

Some experts believe this desire for variety is an evolutionary adaptation that people acquired to get essential nutrients, such as vitamins and proteins, from different food groups. 

"In the long run, we need a good mix of nutrients," Barbara Rolls, director of Penn State's Laboratory for the Study of Human Ingestive Behavior, told Live Science. "Now, where it backfires on us is with the big variety of large portions of calorie-dense foods available to us. The variety does stimulate overconsumption, and so it's potentially contributing to obesity."

When a person consumes a sugary dessert, they also get a hit of dopamine, a chemical in the brain that is associated with feelings of reward and pleasure. If eating dessert is a regular part of your daily routine, "the release of dopamine shifts from after you eat the food to the anticipation of eating the food," Epstein said. This is what constitutes a craving, and it may make you more likely to reach for a slice of pie at the end of a big meal. 

If you are trying to cut back on your post-dinner junk-food fixes, there are a few ways to use sensory-specific satiety to your advantage, according to Rolls. For example, you can keep other sweet foods around, such as fruits, so you are still introducing variety into your diet but in a healthier way. 

"What you need to do … is keep a good variety of healthy, low-calorie [dense], nutrient-rich foods readily available that you enjoy so that when you get the munchies, you have a good variety to choose from," Rolls said.