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: 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!