Dess had an inkling that the rat's taste preferences could be important for research, so she bred it with another rat that was also less into saccharin than average. After tens of generations, she had a line of rats that were so put off by the bitter hints in saccharin that they would drink very little of it despite its overarching sweetness.
Over the years and many experiments, Dess and colleagues found that these rats were also particularly "emotional": more jumpy than regular rats when startled with a loud noise, and more anxious when deprived of food.
In a study published in 2012 in the journal PLOS One, Dess and colleagues showed not only that bitter-sensitive rats were more easily stressed but also that, when competing with others for access to food, they would allow themselves to be shoved aside. In science-speak, they were socially subordinate. In everyday-speak, they were pushovers.
Telltale signs in humans
And it's not just in rats that bitter taste preferences can be a telltale of personality. It seems to apply to humans, too.
About 30 per cent of people can detect the bitter taste of a rather disgusting substance - its scientific name is 6-n-propylthiouracil - even in very low concentrations, and in general these people find bitter taste in foods stronger and less pleasant than other people do.
This sensitivity is genetically based, research has found, and is related in the number of mushroomlike structures on the tongue called fungiform papillae, on which taste buds are perched. Basically, the more fungiform papillae you have, the more sensitive you are to bitterness.
In a 2014 experiment, German and American psychologists showed that, like bitter-sensitive rats, bitter-sensitive people tended to be jumpy, meaning they reacted more strongly than other folks when exposed to a loud noise.
In nature, bitterness tells us that a food may contain toxins, and even such animals as oysters reject bitter-tasting meals. "Bitter taste is a signal for danger. So it is quite logical that people who have heightened sensitivity based on their genes to bitter taste may also have higher sensitivity to other signals of danger in other areas of life, like the social area," explains one of the study's authors, Michael Macht, a professor of psychology at University of Würzburg, Germany.
According to Dess, over the course of evolution, the areas of the brain responsible for dealing with bitter tastes may have been co-opted by higher emotions.
"Taste was one way early organisms had of detecting nutrients and avoiding toxins," she says. "An animal that was attuned to risks would avoid such risks, while another animal would be more attuned to exploring opportunities. Such early pressures on gustation resulted in early changes in how nervous systems functioned - including emotion regulation."
That is why sensitivity to bitterness - as in being able to detect it in low concentrations - may offer hints about personality beyond being jumpy or emotional. A study published in the journal Appetite in January suggests, for example, that there is a link between enjoyment of bitter taste and antisocial personality traits.
In that study, close to 1,000 Americans were given standard personality and taste-preference questionnaires. People who enjoyed foods with bitter notes - such as grapefruit, tonic water, coffee and radishes - were more likely to admit that they enjoyed tormenting people or that they tend to manipulate others to get their way.
"And these effects are not tiny, either," says Austrian psychologist Christina Sagioglou, the study's lead author.
What's more, tastes can affect reactions to daily events, no matter your personality, according to some research.
Imagine you've heard of a politician accepting bribes or of a student stealing library books: How harshly would you judge the offender? According to a 2011 study, that may depend on what you've just tasted: Volunteers who had just taken a gulp of an extremely bitter herbal tonic judged various moral transgressions as far more serious than people drinking nothing but water.
"The findings suggest that judgments involving morality, e.g. jury deliberations, opinions on sociopolitical issues, could potentially reflect and be swayed by what individuals eat and drink," said one of the study's authors, Natalie Kacinik, a professor of psychology at the City University of New York, in an email.
In 2014 Sagioglou and a colleague conducted several experiments in which they showed that tasting bitterness leads to aggression.
An example: After tasting either grapefruit juice or water, students were asked to assess how they'd feel in certain situations - say, if someone kicked the back of their chair repeatedly at the movies. Results showed that the bitter-tasters would react with more hostility and irritation - imagining themselves threatening the annoying moviegoer if he didn't stop - while water drinkers just ignored the bothersome behavior.
Mood sweeteners
So what about sweetness and mood? One 2013 experiment found that thinking of love can make even plain water taste sweeter. A 2015 study suggested that the reverse is also true: Men who were happy because their hockey team had just won a game rated a lemon-lime sorbet as sweeter and less sour than men who had cheered for a losing team.
There's a good explanation for why happy feelings and sweet taste may go together.
Our emotions and taste sensations are connected via hormones and neurotransmitters such as serotonin, noradrenaline and glucocorticoids.
A study done on rats last year showed that receptors for glucocorticoids - primary stress hormones - are located inside taste buds that sense umami and sweetness. If glucocorticoids flood the body, they may inhibit functioning of these taste buds; hence, the dulled culinary pleasures.
On the other hand, a 2006 study found that if it's serotonin (sometimes dubbed the "happy chemical" ) that's released, we become more sensitive to sweet taste and can detect it at concentrations that are even 27 percent lower than before the serotonin release.
The results of some of these taste studies may seem speculative, but Dess believes that once more research is done, "we could make better than random guesses how people will interact with co-workers, how they would respond to different kind of therapeutic interventions, how collaborative they would be, or how empathetic" - just by looking at their taste worlds.
In the meantime, what we already know suggests that you should continue to buy chocolates on Valentine's Day to make your sweetheart feel sweeter (and more romantically inclined) and maybe skip the coffee and grapefruit if you don't want to be judged too harshly.
