The real science behind falling in and out of love

Dry mouth. Butterflies in your stomach. Weak knees.

What better time than Valentine's Day to try to figure out why being in love feels similar to a bad case of the nerves!

To help us, we reached out to a modern day love expert if ever there was one; Helen Fisher, a biological anthropologist, senior research fellow with the Kinsey Institute and author of the book Anatomy of Love: A Natural History of Mating, Marriage, and Why We Stray. Indeed, there are unpleasant side effects that come with falling in love, which Fisher says we can thank evolution for—but, if all goes well, the suffering could pay off, since these feelings are meant to help steer you towards your lifelong mate.

Read on for the questions we asked Fisher as we attempted to demystify the phenomenon of falling in love..

What happens to humans when we fall in love?

The first thing that happens when you fall in love with somebody is they take on special meaning — everything about them becomes special. Three weeks ago, he was just a nice guy in the gym and now, just the way he steps onto the bus is sexy. You become very possessive sexually and socially. You just want this person. You crave them. You can walk all night or talk until dawn. You have elation when things are going well and total despair when they don't write or don't call. You have real bodily reactions — dry mouth, butterflies in the stomach, weak knees. And you're highly motivated to win the person. What people will do when they're in love is absolutely out of this world. It's a drive, like thirst or hunger.

What's happening in our brains?

My colleagues and I have now put over 100 people in a brain scanner, looking to see what happens in the brain. In the first study, I put 17 people into the brain scanner who were madly in love. We found activity in a lot of brain regions. The main brain region where we saw action is called the ventral tegmental area, or the VTA. It's a brain region that actually makes dopamine and sends it to other brain regions, giving you that focus, that motivation, that energy and that craving. What's really going on is it's a basic mating drive that evolved millions of years ago to enable us to focus our mating energy on just one individual and start the mating process.

Are we able to control these responses?

We've evolved a big cerebral cortex and so we do think. We have all kinds of physiological reactions that are instinctual and evolved and chemically-induced, but you can choose to not act on them. Let's say you suddenly fall in love with your boss — you want to keep your job and you know he's in love with his wife and has three children, so you don't express it. Even though we have all these predispositions and they're very powerful strong ones, we still have some control over our actions.

Can you tell me about the three different types of love: sex drive, romantic love and attachment?

The sex drive gets you out there looking for a whole range of partners. It's basically lust. It's a physical craving to have sex with another person and it's orchestrated largely by the testosterone system in both men and women. Romantic love allows you to focus your energy on just one person at a time. It's orchestrated largely by the dopamine system. Attachment allows you to stick with that person at least long enough to raise a child together. It's driven by the oxytocin and vasopressin systems, which give you a feeling of calm and cosmic union and connection and respect.

Is being in love different for men than it is for women?

No. But men do fall in love faster than women do and men fall in love more often than women do.

Lovesickness — is it real?

It sure is. I put 15 people who'd just been rejected in love into the brain scanner. You're still madly in love with somebody who's dumped you, that doesn't change. In fact, frustration attraction can kick in — you can like them even more. We also found activity in the brain region linked with feelings of deep attachment. We found activity in three brain regions linked with craving and one region linked with addiction. It's an addiction — you can't stop thinking about the person.

This interview has been edited and condensed.

Katrina Clarke is a Toronto-based journalist who writes about relationships, health, technology and social trends. Find her on Twitter at @KatrinaAClarke.