Professor Kevin Myers, psychology, studies the psychology of flavor and how it influences our food preferences and choices. His latest research asks about the psychological impact of junk food diets, sugar binging and artificial sweeteners. This Valentine's Day, we asked him why chocolate and other sugary sweets appeal to us so strongly.  

Question: Are we born with a sweet tooth, or is it something we learn?

Answer: It's both. We're born with a very powerful instinctive response to sweet taste. For the most part, mammals innately like sweetness. (The only mammals that don't seem to care about sweet tastes are cats.) In nature, sweet foods like ripe fruit are good for you, and there's a powerful tendency to seek that taste out. At the time of birth that response is very, very strong. There are studies where scientists put sugar on the tongues of newborn infants and get clear smiles. Babies will stop crying. There's a measurable decrease in pain sensitivity.

We innately respond to sweetness with pleasure, but that response dramatically declines with development, and culture and learning determine how much it declines. For children raised in families in cultures where there are a lot of sweets around, the preference for sweetness stays higher. In places where the cuisine doesn't have a lot of sweets in it, that preference drops off dramatically in childhood. Adults still like sweetness, but they like much lower levels of sweetness. Genetics also influences individual differences. Genetic differences in the taste buds cause sugar to taste sweeter for some people.

Question: What happens in the brain when we eat sweets?

Answer: The brain responds in some pretty powerful ways to sweetness. Detectors on the tongue stimulate a reaction in the brain, and the two main brain systems that respond are dopamine and endogenous opiates.

Dopamine is commonly mislabeled as a pleasure chemical, but that's really not accurate. The dopamine-reward system in the brain is not about pleasure; it's about attention. Food distracts people. If you're trying to concentrate on something, and somebody brings a box of doughnuts into the room, those doughnuts will grab your attention.

The pleasure effect comes mainly from the opiates in the brain. These chemicals create a feeling of euphoria and sensory enjoyment, and they relieve pain. In infants, for instance, researchers have actually measured a reduced sensitivity to pain as a reaction to sugar. Nurses taking blood samples can put a drop of sugar on a baby's tongue to reduce pain sensitivity.

Question: What about chocolate? Is there something in it that makes our response extra special?

Answer: Chocolate appears to be an especially powerful stimulus, for several reasons. One is that it's a combination of sweetness and fats. Sweetness and fat are both powerful elicitors of food reward, but the combination of sweet and fat is synergistic. It's more powerful than either on its own.

Also, cocoa is a spice. It's got this complex aroma that enhances the flavor. You can think of it as being like coffee or a chili pepper. It's not necessarily desirable the first time you taste it. The sweetness and fat is, but the complex bitterness and spiciness of cocoa is a little off-putting. But as you get experienced with it, it becomes desirable in and of itself, because you associate the complex aroma with the rewarding stimulation of sweetness and fat.

Question: Does that complexity make it a better Valentine's Day present than, say, flowers?

Answer: Flowers are nice, but you give your mom flowers, and you give people flowers for a funeral. There's a very powerful cultural folklore around chocolate — we think of it as forbidden. The way we talk and think about chocolate and the way it is advertised almost always focuses on the forbidden sensuality of it, which engages this thrill-seeking mechanism in the brain. It's the feeling of doing something that feels good, but that you know is bad. That could be a reason it evokes strong romantic feelings in a lot of people.

Also, there's a reason to spend the few extra dollars to get high quality chocolate for Valentine's Day. The psychologist Paul Rozin makes the point that really good chocolate is made with cocoa butter; in cheap chocolate they take the cocoa butter out and replace it with vegetable shortening. What's appealing about cocoa butter is that it melts at exactly body temperature. That luxurious sensory experience in the mouth — sweetness combined with not just fat, but this special kind of fat that melts just at the temperature of your mouth — is a complex sensory experience that hits a lot of triggers.

Question: What about artificial sweeteners? Can they hit any of the same triggers?

Answer: This is a really controversial area of research. Obviously, the motivation for using artificial sweeteners is that people like sweetness, but sugar has lot of calories in it, and you want the pleasure of the sweetness without the calories. The mouth is actually pretty good at telling the difference between artificial sweeteners and real sugar. Rats and people both perceive artificial sweeteners as tasting okay, but not great. They tend to have a chemical taste to them. The controversy, and the reason people are interested in studying this, is that a variety of evidence suggests that using artificial sweeteners can actually cause weight gain.

One explanation that's been proposed for this — and this is something I'm working on with my students now, too — is the idea that ordinarily, we use the taste of something to predict what its nutritional consequences are going to be. If you eat a food and then get sick, you learn that that flavor signals danger, and you don't like it anymore. We pick up on the relationships between foods that have a certain flavor and how satisfied we tend to feel after eating them. This happens not at a cognitive level but on a more automatic, Pavlovian level. We use that information to adjust our likes, dislikes and eating habits.

The potential problem with artificial sweeteners is that they interfere with the brain's ability to do that. They can destroy the relationships that exist in natural food between sweetness and calorie content. You're getting the stimulation of a sweet taste, but it's not followed by the calories, so in your experience, sweetness sometimes means calories, and sometimes doesn't. This leads to so much confusion that the brain gives up on using how sweet something tastes as a cue for how filling or satisfying it should be, or as a cue for how to metabolize it. Eventually the brain stops relying on sweetness as a signal, which leads to overeating.