A recent study suggests that people who prefer foods and drinks with bitter tastes are more likely to be psychopaths. Specifically, gin and tonics were one of the bitter drinks preferred by those with antisocial personality traits. In this case, "antisocial" doesn't mean they hate parties, it means they're more likely to be selfish, narcissistic or sadistic.
The study, which will be published in the journal Appetite, measured preferences for sweet, sour, salty, and bitter foods. They then compared those preferences with participants' answers to questions like "I have threatened people I know" and "I tend to manipulate others to get my way." Evolutionarily speaking, bitter foods are associated with toxins, so psychos may simply like the thrill of eating and drinking things that cause a brush with death (or give you a sweet buzz).
So, if the person you're with orders a gin and tonic, don't be surprised if they try to manipulate you into picking up the tab. If they do both those things and also mention how much they like your skin, look for the nearest exit.