Taste of a Kiss

The Romantic Notion:

"Ah, the kiss, the connection of two souls, the action that binds both young and old lovers for eternity. The romantic tension ... the passion ... the intimacy ... the lust ... the burning desire ... the hunger for more ..."

Science Says:

There's a cluster of chromosomes present in the body called MHC (major histocompatibility complex) that controls part of your immune system. And when you're out looking for someone to breed with, MHC is probably the most influential aspect of a partner that you didn't know you were judging. MHC controls your ability to fight off infection, and if you breed with someone whose MHC is similar to your own, the pregnancy is less likely to take. Finding someone whose MHC is different means a more diverse immune system for the child. 

In other words, evolution supports getting strange with strangers.

In fact, according to a report in Psychology Today, the scent of MHC might be the second-most-important factor in determining how attractive a woman finds a potential mate. While you might not realize you're secretly judging someone you just met on how well your child would be able to fight off a cold, that's what's happening. Your body tries to instinctively make sure that each potential partner has the sexual compatibility seal of approval. Now how does it do that?

MHC is present in both pheromones and saliva, meaning that to really detect whether a partner is suitable, one must be in close proximity (to smell the MHC), and there must be an exchange in saliva (to taste the MHC). Now, what usually happens when these two events are placed together? That's right, ladies and gentlemen -- the kiss is in fact a taste test. We've adapted the behavior to make sure we find someone with whom our chemicals match up.

"Wow! We both taste like shame."

So all that stuff about loving someone's soul? You could go out and find someone who shares your interests, reads the same manga, orders the same pizza, gets along with your parents and even gets the "subtle intelligent humor" that you (and only you) understand. All it would take is the taste of that person's saliva for that interest to go from "I want to make love to you right now! I don't care if the kids I'm babysitting are watching" to "I like you, but more like a brother."

  

If she's been on birth-control pills the whole time you dated, there's a chance you're both being tricked into marrying exactly the wrong person by your own bodies. 

When a woman is actually pregnant, her body decides, "It's not like I can get more pregnant," and it stops doing a bunch of the things it normally does. The pill basically uses hormones to convince a woman's body that it's already pregnant. The woman doesn't want to get pregnant, her body thinks it's pregnant, everyone's happy.

Or at least, they would be if it weren't for that pesky MHC stuff controlling who you can fall in love with. Just as a pregnant woman might find herself suddenly craving food she used to find repulsive, her taste in MHC undergoes a polar reversal. She's no longer attracted to people with MHC that is dissimilar to hers, and way more attracted to men with similar MHC. From an evolutionary perspective, this was probably so that women would want to spend more time around family members in a protective environment rather than out at a bar trying to get laid again. In a modern context, it's probably why pregnant women so often want to murder their husbands with a meat cleaver: He no longer smells like her type, and it's far too late for that. 

A woman on the pill gets exactly the same effect, without the belly or the appearance of the boob fairy. For the entire time she's on the pill, a woman will prefer people with MHC that is similar to her own. This is why some psychologists believe that the high divorce rate in modern times can be blamed on the pill. Two people can be dating for years, thinking they're meant for each other when in reality, their MHC is the exact opposite of compatible. Of course, they only find out when they're ready to hatch one and she goes off the pill, which of course is often way, way too late.