by Katherine Wu
figures by Tito Adhikary

In 1993, Haddaway asked the world, “What is Love?” I’m not sure if he ever got his answer – but today, you can have yours.

Sort of.

Scientists in fields ranging from anthropology to neuroscience have been asking this same question (albeit less eloquently) for decades. It turns out the science behind love is both simpler and more complex than we might think.

Google the phrase “biology of love” and you’ll get answers that run the gamut of accuracy. Needless to say, the scientific basis of love is often sensationalized, and as with most science, we don’t know enough to draw firm conclusions about every piece of the puzzle. What we do know, however, is that much of love can be explained by chemistry. So, if there’s really a “formula” for love, what is it, and what does it mean?

Total Eclipse of the Brain

Think of the last time you ran into someone you find attractive. You may have stammered, your palms may have sweated; you may have said something incredibly asinine and tripped spectacularly while trying to saunter away (or is that just me?). And chances are, your heart was thudding in your chest. It’s no surprise that, for centuries, people thought love (and most other emotions, for that matter) arose from the heart. As it turns out, love is all about the brain – which, in turn, makes the rest of your body go haywire.

According to a team of scientists led by Dr. Helen Fisher at Rutgers, romantic love can be broken down into three categories: lust, attraction, and attachment. Each category is characterized by its own set of hormones stemming from the brain (Table 1).

Table 1: Love can be distilled into three categories: lust, attraction, and attachment. Though there are overlaps and subtleties to each, each type is characterized by its own set of hormones. Testosterone and estrogen drive lust; dopamine, norepinephrine, and serotonin create attraction; and oxytocin and vasopressin mediate attachment.
Table 1: Love can be distilled into three categories: lust, attraction, and attachment. Though there are overlaps and subtleties to each, each type is characterized by its own set of hormones. Testosterone and estrogen drive lust; dopamine, norepinephrine, and serotonin create attraction; and oxytocin and vasopressin mediate attachment.

Let’s Get Chemical

Lust is driven by the desire for sexual gratification. The evolutionary basis for this stems from our need to reproduce, a need shared among all living things. Through reproduction, organisms pass on their genes, and thus contribute to the perpetuation of their species.

The hypothalamus of the brain plays a big role in this, stimulating the production of the sex hormones testosterone and estrogen from the testes and ovaries (Figure 1). While these chemicals are often stereotyped as being “male” and “female,” respectively, both play a role in men and women. As it turns out, testosterone increases libido in just about everyone. The effects are less pronounced with estrogen, but some women report being more sexually motivated around the time they ovulate, when estrogen levels are highest.

Figure 1
Figure 1: A: The testes and ovaries secrete the sex hormones testosterone and estrogen, driving sexual desire. B and C: Dopamine, oxytocin, and vasopressin are all made in the hypothalamus, a region of the brain that controls many vital functions as well as emotion. D: Several of the regions of the brain that affect love. Lust and attraction shut off the prefrontal cortex of the brain, which includes rational behavior.

Love is its Own Reward

Meanwhile, attraction seems to be a distinct, though closely related, phenomenon. While we can certainly lust for someone we are attracted to, and vice versa, one can happen without the other. Attraction involves the brain pathways that control “reward” behavior (Figure 1), which partly explains why the first few weeks or months of a relationship can be so exhilarating and even all-consuming.

Dopamine, produced by the hypothalamus, is a particularly well-publicized player in the brain’s reward pathway – it’s released when we do things that feel good to us. In this case, these things include spending time with loved ones and having sex. High levels of dopamine and a related hormone, norepinephrine, are released during attraction. These chemicals make us giddy, energetic, and euphoric, even leading to decreased appetite and insomnia – which means you actually can be so “in love” that you can’t eat and can’t sleep. In fact, norepinephrine, also known as noradrenalin, may sound familiar because it plays a large role in the fight or flight response, which kicks into high gear when we’re stressed and keeps us alert. Brain scans of people in love have actually shown that the primary “reward” centers of the brain, including the ventral tegmental area and the caudate nucleus, fire like crazy when people are shown a photo of someone they are intensely attracted to, compared to when they are shown someone they feel neutral towards (like an old high school acquaintance).

Finally, attraction seems to lead to a reduction in serotonin, a hormone that’s known to be involved in appetite and mood. Interestingly, people who suffer from obsessive-compulsive disorder also have low levels of serotonin, leading scientists to speculate that this is what underlies the overpowering infatuation that characterizes the beginning stages of love.

The Friend Zone

Last but not least, attachment is the predominant factor in long-term relationships. While lust and attraction are pretty much exclusive to romantic entanglements, attachment mediates friendships, parent-infant bonding, social cordiality, and many other intimacies as well. The two primary hormones here appear to be oxytocin and vasopressin (Figure 1).

Oxytocin is often nicknamed “cuddle hormone” for this reason. Like dopamine, oxytocin is produced by the hypothalamus and released in large quantities during sex, breastfeeding, and childbirth. This may seem like a very strange assortment of activities – not all of which are necessarily enjoyable – but the common factor here is that all of these events are precursors to bonding. It also makes it pretty clear why having separate areas for attachment, lust, and attraction is important: we are attached to our immediate family, but those other emotions have no business there (and let’s just say people who have muddled this up don’t have the best track record).

Love Hurts

This all paints quite the rosy picture of love: hormones are released, making us feel good, rewarded, and close to our romantic partners. But that can’t be the whole story: love is often accompanied by jealousy, erratic behavior, and irrationality, along with a host of other less-than-positive emotions and moods. It seems that our friendly cohort of hormones is also responsible for the downsides of love.

Dopamine, for instance, is the hormone responsible for the vast majority of the brain’s reward pathway – and that means controlling both the good and the bad. We experience surges of dopamine for our virtues and our vices. In fact, the dopamine pathway is particularly well studied when it comes to addiction. The same regions that light up when we’re feeling attraction light up when drug addicts take cocaine and when we binge eat sweets. For example, cocaine maintains dopamine signaling for much longer than usual, leading to a temporary “high.” In a way, attraction is much like an addiction to another human being. Similarly, the same brain regions light up when we become addicted to material goods as when we become emotionally dependent on our partners (Figure 2). And addicts going into withdrawal are not unlike love-struck people craving the company of someone they cannot see.

Figure 2: Dopamine, which runs the reward pathways in our brain, is great in moderate doses, helping us enjoy food, exciting events, and relationships. However, we can push the dopamine pathway too far when we become addicted to food or drugs. Similarly, too much dopamine in a relationship can underlie unhealthy emotional dependence on our partners. And while healthy levels of oxytocin help us bond and feel warm and fuzzy towards our companions, elevated oxytocin can also fuel prejudice.

The story is somewhat similar for oxytocin: too much of a good thing can be bad. Recent studies on party drugs such as MDMA and GHB shows that oxytocin may be the hormone behind the feel-good, sociable effects these chemicals produce. These positive feelings are taken to an extreme in this case, causing the user to dissociate from his or her environment and act wildly and recklessly. Furthermore, oxytocin’s role as a “bonding” hormone appears to help reinforce the positive feelings we already feel towards the people we love. That is, as we become more attached to our families, friends, and significant others, oxytocin is working in the background, reminding us why we like these people and increasing our affection for them. While this may be a good things for monogamy, such associations are not always positive. For example, oxytocin has also been suggested to play a role in ethnocentrism, increasing our love for people in our already-established cultural groups and making those unlike us seem more foreign (Figure 2). Thus, like dopamine, oxytocin can be a bit of a double-edged sword.

And finally, what would love be without embarrassment? Sexual arousal (but not necessarily attachment) appears to turn off regions in our brain that regulate critical thinking, self-awareness, and rational behavior, including parts of the prefrontal cortex (Figure 2). In short, love makes us dumb. Have you ever done something when you were in love that you later regretted? Maybe not. I’d ask a certain star-crossed Shakespearean couple, but it’s a little late for them.

So, in short, there is sort of a “formula” for love. However, it’s a work in progress, and there are many questions left unanswered. And, as we’ve realized by now, it’s not just the hormone side of the equation that’s complicated. Love can be both the best and worst thing for you – it can be the thing that gets us up in the morning, or what makes us never want to wake up again. I’m not sure I could define “love” for you if I kept you here for another ten thousand pages.

In the end, everyone is capable of defining love for themselves. And, for better or for worse, if it’s all hormones, maybe each of us can have “chemistry” with just about anyone. But whether or not it goes further is still up to the rest of you.

Happy Valentine’s Day!

Katherine Wu is a third-year graduate student at Harvard University. She loves science with all of her brain.

Further Reading

  1. For a long-form human interest story on love, see National Geographic’s coverage of “True Love”
  2. For a very in-depth (and well-done!) introduction to the brain and its many, many chemicals, check out the NIH’s Brain Basics page
  3. For the New York Times’ take on falling in love with anyone, ask these 36 questions

221 thoughts on “Love, Actually: The science behind lust, attraction, and companionship

  1. In my opinion,there is never a clear definition of love, and all we can do is redefine it after integration. It can’t cover everything, in other words, we can’t define a love that makes everyone believe.

  2. The chemical argument for love, while not entirely unfounded, has a glaring weakness beyond the obvious incomplete picture of it.
    Every chemical related, while certainly associated with the experience of love, has more than that singular function.
    For example: You could load a subject up with all the hormones related to attraction without any greater measurable chance of them becoming attracted to you if they weren’t already.
    This article was upfront with the fact that the chemistry is only part of the equation, but based on current understanding, the attraction comes first which triggers the chemicals we can measurably associate with attraction. Therefore it would seem the chemical components we point to are (for the most part) the consequencial effect, not the cause.
    It seems thus far science has it backwards.
    Perhaps the missing link chemical formula simply has yet to be discovered, but with all we think we understand, we are no closer to a love potion than the primative witch doctors were thousands of years ago.

  3. The article adequately addresses the individual but fails to address the case of mutual attraction and bonding considered triggered and regulated (at least partially) by pheromones. And what causes attraction to fade and eventually wither away – is it always just a decreasing production of hormones? If so, what cause the decrease? There still seems to be some missing ingredients in a purely chemical treatment of the subject.

    1. I’m not a scientist but I expect attraction fades because it is not practical to be so obsessed with a potential mate for a long time, a lot of other things to think about to survive, genetically those that stayed focused on attraction would have died out. Most animals I’ve seen on TV only spend some of their time in mating behavior but it takes over completely over that time, not practical to be like that all the time.

      1. Whilst I agree with the general sentiment of your comment and accept the premise that there is some impracticality in having one life-long mate. I am inclined to disagree that there aren’t benefits to sexual monogamy, especially humans; for example reduced promiscuity certainly decreases the chances of contracting sexually transmitted infections (physical survival), and as society has evolved beyond stone tools and cave paintings, having a single partner and marrying, as a behaviour, gives one access to greater social survival through financial interdependence, social status, etc etc… human relationships and attraction is much more complicated that other mammals and, as such, comparisons between animal behaviour and human behaviour may not be the most compelling argument as to why attraction comes and goes.

        1. Interesting, humans are more complicated than other animal, we are animals with reproductive instincts but culture also influences and sometimes overrides instincts . I wasn’t thinking about the issue of monogamy for life, many factors probably influence this, human children require a long period of upbringing- probably better to have their biological parents doing it than a series of strangers. If everyone was promiscuous there might be so many fights over mates we could not have civilized lives. While attraction, especially sexual attraction and definitely infatuation, fade in my experience, they do not necessarily go away completely, you can remain attracted to the same person for life, but I’d say most couples who stay together happily are also good friends, not exactly the same as physical attraction. As I say I’m not a scientist, just speculation.

    2. Excellent effort, Katherine! Genuinely a tale well told…You are also a true and faithful follower of the Science-can-explain-everything doctrine (or the “I believe in Science, not God!” of my then-four year-old son, despite my protesting, and his Science-trained grandparents, and his Maths-trained father, that it’s possible to ‘believe’ in both.)

      Biological reductionism reduces a human being (or any living thing) to a pre-programmed machine. That is what Helen Fisher and others of her ilk are providing. Not that there isn’t some value in what’s being discovered, but it is to fall into the fallacy that Science can best explain these things when in fact, Science can only, at best, be a minor player at the edges… In my six years in medical research I came across very few scientists who appreciated the Humanities, especially the expressive arts, as holding answers that Science had little to nothing to say about. Truth and reality are not the sole province of Science, though ‘it’ likes to insist on that hegemony… Chemical messengers are wonderful proteins, and it’s fascinating and mind-blowing what hormones and the like can do (I trained initially in Biochemistry and went on to Immunology, and to those clever little cytokines). The eukaryotic cell is an amazing feat of breath-taking ingenuity, as is the human body. The human brain, or that of most of the animal kingdom, is never going to be understood in its entirety, because there are real limits to the power of Scientific thinking and discovery. The only way through may be quantum theory/physics etc because that allows, for instance, for divergence and in so doing, there is room for mystery rather than a rush for a linear solution. Of course we try and understand this mystery, explore it as best we can, but we may have to accept that not everything is soluble. I love Science and am a great believer in it – embracing it generally for its invaluable truths (I don’t ‘believe’ in it however, because eg Climate Change is not a matter of belief but acknowledgement). Wholehearted biological reductionism (and its bedfellow, molecular determinism – think Richard Dawkins) really misses the point that humans, and animals and life in general is far more than what can be quantified and measured, tho’ there are a lot of useful things that can be measured! And human love is far more than what can be quantified and measured through associative links. I think of it as drawing a 2D representation of something called love and then insisting that 2D representation explains the 3D love-in-lived-reality. It may offer some insights, some interesting observations, but it doesn’t “do” depth – it’s like have just one eye, not two.
      If we really want to understand what human love is like we are better off turning to the poets.. and the musicians… and the novelists … and the playwrights … and the dancers… They speak so much more directly to the actual experience of what love is than the chemical messengers that turn this or that “light bulb” on in the complex circuitry of the brain.

      1. Fantastic response Ruth!

        We (as humans) are limited in our understanding. Many aspects of our existence will likely remain beyond our abilities to measure and quantify. That said, attempts (such as Katherine’s) to narrow the gaps of our incomplete understanding should be applauded.

      2. Human love is far more than can be quantified and measured now, but I believe the data is insufficient to conclude that it will never be quantifiable. God? Maybe. Biological determinism? That’s a ‘maybe’, too, IMO.

  4. well for me, love is just like a flower that you need to water, and time will come that it will produce the flowers and the seeds itself…it’s always needing water and sunlight’s.

  5. Hello. I read the article and it was quite interesting. One of the main things I want to question though is what part of attachment does vasopressin play in? It wasn’t clearly mentioned in the text, so I couldn’t quite get it. Its still an amazing study.

  6. Very good article, but is it atachment real love, the unconditional, higherlove we read and hear from great masters of all times?! Attachement can be it’s start, like a relatipnship evolving from lover’s to friend’s and friendship to friendliness

    1. I entered into a relationship like this, I didn’t have a name for it but he told me. It was a awesome year that I will cherish.

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