Why Do We Love? The Science of Our Oldest Drive

Since the days when our ancestor Australopithecus first sauntered across the African savannah, love has been one of the most cherished and enduring mysteries of nature. It’s hard to define, but we sure know it when we see it (or feel it).

We’re physiologically capable of mating with any member of the opposite sex, after all, but that doesn’t mean we want to. Some sense of emotional and sexual urgency drives us towards that special someone — even animals exhibit behaviors indicating that they’re attracted to specific mates.

The reason love is so hard to pin down scientifically isn’t just because it takes so many different forms (between friends, parents and offspring, pets, etc.) but because it’s not so much a part of the evolutionary urge as we first assumed. Some romantic love — like homosexuality, or the schoolyard crush we experience long before sexual maturity — serves no reproductive purpose at all.

But following the work of behaviorists, neuroscientists, and evolutionary psychologists, we might be on the verge of unpacking the deepest, oldest drive known to humanity.


As with every other state of being, love’s origin has to do with the balance of hormones and neurotransmitters in the brain. In “Survival of the Nicest: How Altruism Made Us Human and Why It Pays to Get Along,” science author Stefan Klein tells us what the brain hormones vasopressin and oxytocin (collectively known as vasotocin) do.

Both are produced in the posterior pituitary gland and studies on animals have shown they play a part in pair bonding and social hierarchy. Oxytocin is the chemical that contributes to feelings of elation during an orgasm, and the female brain also gets a burst of it during labor — which is thought to help the mother bond with the developing baby.

The other ingredients of love we find in the neurological soup include dopamine, norepinephrine, testosterone, serotonin and lots more associated with reward, pleasure, attachment and its suppression (which we’ll hear more about later).

Biological anthropologist, author, and love expert Helen Fisher performed a unique set of experiments when she put people who were in love (both of the requited and unrequited varieties) in functional MRI machines to see what their brains were doing in each case.

The results, coupled with rudimentary knowledge of how certain neurohormones affect our moods, let her put together a unique picture of how both sexual attraction and love evolve. She characterizes a love affair in phases where different chemicals affect our behaviors and feelings.

Testosterone is prevalent in the lust phase, a phase similar to having an itch or being hungry — when it is an almost purely physical need. When we fall in love, romantic attraction is produced in part by high levels of dopamine and norepinephrine as well as low levels of serotonin. If we have a casual lover or one-night stand, we don’t really care who else our partners are with, but falling in love makes us sexually possessive and thus incites the craving to be together on both an emotional as well as sexual level.

If all goes well and our yearnings are returned, we move into the attachment phase and vasotocin plays a part in generating feelings of calm and security. This, in turn, leads to the pair bond behavior evolution says we were put on earth for — giving offspring a decent chance to carry our genes into the future.

Fisher’s findings have also convinced her that the drive for romantic love is as instinctive as that for food and security, and another quirk of the human nature indicating that the brain activity involved in love is the same regardless of the subject. The difference between loving your husband or wife, your dog and your car, might only have a slight degree of difference.

All of which means that love, of any kind, might be made of the same few discrete chemical parts, merely arranged in different concentrations at different times.


If that last sentence makes you bristle, spare a thought for researchers trying to explain love logically. Science has long faced criticism for oversimplifying, and, in an increasingly secular and brutally pragmatic world, love is one of the few areas about which we still get to enjoy a little bit of mystery and magic.

But before you resolve not to pay attention to any findings, keep a few things in mind. First, humans aren’t segmented flatworms or bacteria. We’re the sum of much more than instincts or hormones —  built by our own individual experiences and brain plasticity, and shaped by hardwired and cultural behaviors. Science might never really disentangle it all.

We also shouldn’t sandbox something even as seemingly simple as a brain chemical. We tend to equate testosterone with aggression and serotonin with depression (thanks to its prominence in depression treatments), but they’re all part of a multistranded web in which they serve different functions and purposes. For example, vasopressin’s primary purpose involves retaining water in the body and constricting blood vessels. Only thanks to recent studies do we know that it plays a role in social behavior and sexual motivation.

But here’s the kicker: knowing a bit more about how love works in the brain gives us the opportunity to “cheat” our nervous system a little and have the kind of love we want. According to Fisher’s work, the vasotocin levels associated with a long-term partnership affect hormones that play a part in romantic love and sexual arousal, which partly explains the frustrating way romance seems to fade over the years.

The reason a new love affair (as opposed to just casual sex) is so all consuming is because this neurococktail of hormones makes us want to spend months in bed together and floods us with feelings of reward and satisfaction. Doing something novel like going on a holiday or trying a new restaurant with your long-term spouse can trigger the same chemical spikes responsible for romantic love.

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