There's nothing quite like the feeling of new love. But what's really going on with our nervous systems when we get butterflies? And what can our biochemistry tell us about compatibility?

All sorts of chemicals affect our feelings and behaviour, but by far the most prevalent are hormones. Hormones are just chemicals that carry messages in the body, and in this blog post we discuss several of them – plus a few neurotransmitters you've probably heard of already. But more on those later...

Oestrogen and testosterone

Perhaps the most famous examples, oestrogen and testosterone (often referred to as sex hormones) both play important roles in attraction for both males and females.

In general, increases in oestrogen for women are correlated with increases in sexual desire and attraction. This is most notable during the ovulation phase of women's menstrual cycles, i.e. the part of each month during which they're able to become pregnant.

Significantly increasing testosterone levels also tends to increase women's sexual desire, although the testosterone levels required are higher than would ever be found naturally. Testosterone therapy also tends to support oestrogen therapy more than it works independently, and increasing testosterone by such a degree can lead to "somatic virilisation," i.e. a lowering of the voice, the growth of facial hair, and enlargement of the clitoris (part of why testosterone is used to help address gender dysmorphia in trans men).

Another relevant hormone in women is progesterone. Best-known for its role in hormonal contraception, progesterone tends to have a negative effect on female sexual desire, both for individual women at various points in their menstrual cycle (during which both oestrogen and progesterone naturally fluctuate) and between women with different levels of progesterone.

In men, the picture is more straightforward: testosterone governs the majority of male sexual desire. This doesn't mean that how much testosterone a man has determines how powerful his sex drive is, however: the testosterone has to be bioavailable (available for the relevant chemical reactions in the body, which not all hormones are all the time) to drive libido. Levels of testosterone are also connected to erectile function, although many of the behavioural effects traditionally associated with testosterone are usually misunderstood.

Oxytocin and vasopressin

But love and attraction aren't just about sex hormones. Oxytocin and vasopressin are two more hormones that play important roles in our romantic behaviour. Both are produced mainly in the hypothalamus, a small gland in the lower middle of your brain.

In some pair-bonding mammals, oxytocin and vasopressin serve to enhance pair bonding after sex, possibly through interactions with dopamine (more on that later).

This is particularly true of oxytocin in females and vasopressin in males, although the reasons for the difference aren't well-understood. We do know that upon becoming fathers, male primates tend to see an increase in their number of certain vasopressin receptors, which might support the idea that vasopressin plays a role in pair bonding. In humans, some research has also found a link between marital happiness and the genes that govern vasopressin receptors.

Oxytocin also seems to increase trust, empathy, and generosity, suggesting that it too has a role in the more intimate parts of our social lives. Although oxytocin is produced naturally in the body, it can also be manufactured and used as a drug. In that form, it's recently been found useful for treating social anxiety and autism. It also plays important roles in our sexual behaviour, including by facilitating arousal.

But its most intriguing task is in the formation of new romantic relationships. Oxytocin levels in early-stage lovers are particularly high – significantly higher than those in single people – and these levels can even predict relationship success, as measured by how attentive couples are to each other, and how closely their emotional states mirror each other.

This is very similar to the role played by oxytocin in new parents. The hormone is most famous for facilitating deep, stable bonds between women and their newborns, and recent research shows that a similar effect exists for fathers. Higher levels of oxytocin in parents tend to predict higher levels of oxytocin in infants, along with more sensitive and higher-quality parenting.

There's also some evidence that oxytocin helps men in monogamous relationships keep their distance from attractive women.

Adrenaline and cortisol

Adrenaline, also called epinephrine, is a hormone secreted by the adrenal glands. Its main job is to make sure that our basic bodily functions (like breathing and blood flow) are working properly, particularly when we're under stress. When our 'fight-or-flight' response is triggered, adrenaline increases blood flow to our muscles, including our heart.

But it also plays a role in romantic attraction. Adrenaline makes us feel physically (not necessarily sexually) aroused, or activated, and it's very common to mix that up with romantic attraction: in fact, several studies have found that after scary experiences or doing lots of exercise – both activities that produce adrenaline – we're much more likely to report being attracted to other people.

One study, conducted in 1974, positioned a beautiful female interviewer on two kinds of bridge: a normal bridge to begin with, and on a subsequent occasion, a "fear-inducing" suspension bridge. 85 male passersby were interviewed by her, and she invited them to contact her after the interview in case they had any questions.

Those interviewed on the rope bridge were significantly more likely to contact the interviewer after the event. (A male interviewer was also used with a control group, in which no significant differences between the bridges were found).

One of the reasons adrenaline can make us feel attracted to each other is that we misattribute our physical excitement to romantic attraction. That's because when we are in love, we tend to be excited and produce adrenaline – making us more alert, more attentive, and less hungry (because blood is rushing to our muscles instead of our stomach, which is what causes the "butterflies" feeling).

It's not just adrenaline that dims our appetite when love comes knocking. Cortisol is another stress hormone produced by the adrenal glands, and you probably know what it feels like: it's the hormone that wakes you up in the morning, and it helps with bodily functions from metabolising sugars to lowering inflammation.

In relationships, lovers typically see lower cortisol levels than average – perhaps because romantic fulfilment lowers stress levels. But the opposite is true at the start of romantic relationships: during dating, our cortisol levels tend to increase (at least until we find someone we like), particularly if we place a high value on romantic relationships. This is understandable: the higher the stakes, the more nervous we're likely to be.

Serotonin and dopamine

Most famously associated with happiness, serotonin and dopamine are both neurotransmitters (chemicals that send signals in your brain) that help regulate mood and social behaviour.

Both are believed to play a crucial role in creating attraction, partly because they increase feelings of reward associated with positive experiences. But when we're falling for someone, we usually see serotonin levels decrease. This may be why lovesickness isn't always fun: we're lusting after something we don't (yet) have, and at worst this can make us obsessive and miserable.

Dopamine activity, however, increases when we see our partner (and possibly other loved ones). This is a well-replicated finding, and associated with the idea that dopamine's function is at least partly to guide us towards things that make us happy.

Endorphins

"Endorphin" is a contraction of "endogenous morphine," and that should give you a pretty good idea of what these little chemicals do for us. They're the body's natural painkillers, and they're produced by all sorts of activity: laughter, exercise, and even meditation.

They're also released (along with oxytocin) during sex, which is one of the reasons it's sometimes recommended to help treat period cramps. It's hypothesised that endorphins help stimulate the production of other hormones involved in reproduction and bonding, too, which might also explain why exercise makes us more attracted to other people. We also produce more endorphins when we stop drinking alcohol, discontinue drugs, and stop smoking.

That said, we also know that a number of opiods produced outside the body seem to have the opposite effect, dampening libido, reducing testosterone in men, and limiting orgasm. Researchers suspect that these conflicting effects are probably to do with doseage and frequency of exposure.

Pheromones

Pheromones are a bit different from the other chemicals we've discussed so far, but no writing about the biochemistry of love would be complete without a quick overview of what they are and how they work.

While the other substances in this article are used inside the body, pheromones are chemicals secreted by it into the environment. More specifically, they're chemicals designed to trigger some kind of social response; a bit like wearing a nice perfume to be perceived as pleasant to be around.

In non-human animals, we know that pheromones can serve several functions: attracting mates, marking paths, signalling availability for sex, indicating territory, repelling predators, inducing suckling, and calming or aggravating members of the same species.

Humans also secrete pheromones, and we're able to detect them with our noses. But their effects on our behaviour are a subject of intense debate, and no peer-reviewed study has yet proved that pheromones affect human behaviour.

Nonetheless, the fact that our bodies start producing pheromones during puberty suggets they may play a role in our sexual selection. In particular, they may guide us (and particularly women, who have better senses of smell than men) towards partners with different immune systems to our own, which in turn could maximise the potential benefits to offspring.

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