- I'm here at Austin's most famous Instagram mural to ask what's the stupidest things people have done for love?

- I met this girl one time, and after that I said, "How about we go to Arizona?"

And here we are two years later.

- [Presenter] Did it work?

- [Brown Top] It did.

- Definitely drive to Tennessee, like 11 at night.

Definitely worked out, yeah.

- [Presenter] She's standing right there.

You have to say that.

- [Blue Shirt] Yeah.

Yeah.

(faint distant music) - When we do things in the name of love, it feels like we're making a choice, right?

But what if it isn't a choice?

What if our brains made us do those dumb things for a reason?

Well, science has figured out what love is actually for, and there's this one secret that completely changed how I look at the biological purpose of love, and it explains everything we do in the name of romance.

(bright music) Love is a mysterious thing, and it makes people do absolutely bananas stuff like moving across the country for someone you've just known for a few days, that tattoo you don't like to talk about, or how you gave her that mint-condition first-edition Charizard card that she will definitely later lose.

I didn't make these up.

According to the internet, these are all things that otherwise normal people have done for love.

If we step back though and take a rational look at many of the things we do for love, they're stupid.

There's no better way to say it.

So why does a species that's smart enough to build rockets, or unlock the secrets of the brain, or write "Romeo and Juliet" like to throw gum in the gears of life in the name of love?

Why did evolution forget to make us smart when it comes to love too?

Well, what if doing irrational, illogical, crazy things in the name of love isn't a mistake?

It might be that love is supposed to make us stupid.

This might be the answer to why love exists at all, but to understand why that is, we need to look at what love really is and, well, that happens in the brain.

Look, I know it's not exactly poetic to talk about, but love, just like any behavior that we do or emotion that we feel, it's something that happens in our brains.

And that's thanks to chemicals flooding different parts of our brain and turning some of them on and others off for very carefully evolved reasons.

In the early 2000s, researchers decided to put people who were madly in love into an fMRI brain scanner, and then they showed them pictures of the people that they were head over heels for.

What an fMRI is looking for is which parts of the brain the activity turns up or turns down.

And when people saw pictures of their beloved, these areas lit up.

These are the same parts of the brain that light up when someone takes drugs.

It's our brain's reward circuit.

Our brain floods these areas with the neurotransmitter dopamine so that we feel good and that we'll be motivated to do that thing again.

If your brain in love is running the same program it runs on drugs, no wonder it feels like we're out of our minds.

But there's clearly something even deeper happening because, sure, romantic love gives us a rush of pleasure, but it's more than that too.

It's attraction, it's obsession, it's comfort and attachment.

A brilliant researcher named Helen Fisher realized this, and she created this pretty genius framework for what love is.

According to her, love isn't one thing or one mental state.

It's three separate systems.

One, lust or infatuation.

It's driven by hormones like testosterone and estrogen.

Hormones that drive those fiery feelings of desire.

And if you're sitting there wondering, "Desire for what?"

well, go ask your parents because I ain't your dad.

Second comes the system that drives romantic attraction.

Now, lust, that's a pretty broad feeling, your brain is just looking for someone, but romantic attraction is focused like a laser beam.

I want this specific person.

This is where the dopamine floods kick in.

Your brain rewards you to keep your attention right here.

But that's not all love is either.

There's a third system, attachment.

Once we've focused in on that person, our brain is bathed in other chemicals, like oxytocin, vasopressin.

This is where those feelings of attachment come from.

A calm sets in, a comfort.

Something that says, "I want to stay with this person."

What's interesting is that these systems probably evolved separately, which means that sometimes they can point at different people at the same time, which has caused a lot of misery and strife frankly.

But as far as we know, other animals, they only have systems one and three.

They have that sexual desire, a desire to mate, and plenty of animals have feelings of attachment, like the kind of attachment that makes offspring bond with their mother.

But that second one, the obsessive focused romantic attraction that we associate with falling in love.

Only humans seem to do that.

That's where we get weird.

And when you're falling in love with someone, lots of other stuff starts going haywire inside your skull, like your levels of serotonin, another transmitter.

They drop in a way that honestly looks a lot like OCD.

That would explain why you can't stop thinking about them, why they dominate your attention.

Chemical compulsion, it's not very poetic.

But I gotta give it to you straight, okay?

Your brain has essentially marked this person as essential to your survival and it will not let you forget about them.

Other brain scan studies of people in love showed that it also deactivates some parts of your brain like ones that we use for pretty important stuff, like judgment, and critical thinking, and predicting whether what we're about to do might have negative consequences.

So love pushes the feel-good parts and it basically turns off your brain's fact checking department, and the part of your brain that might, I don't know, notice red flags.

It's like love actually makes you blind to good judgment.

When evolution was crafting this whole mess called love it apparently didn't want you second guessing yourself the whole time or really thinking straight in general.

And the reason why is probably the most important part of this video, so pay attention.

Okay, remember natural selection keeps traits that help pass our genes into the future and it eliminates ones that get in the way of that.

So why the heck would natural selection build something like love?

What is in it for us and the future of our genes?

Sticking with one mate is actually pretty rare on our branch of the family tree.

Only about 3 to 5% of mammals form lasting pair bonds.

Think about it.

If natural selection was only about getting as many copies of your genes into the future as possible, then investing in one partner and just one or a few offspring, that seems like a bad bet.

So why did humans evolve to do exactly this?

Next to, I don't know, a baby panda, human infants may be the most helpless mammal babies on earth.

They need constant care for years.

It's a lot.

I mean, you think I look older these days 'cause I'm making YouTube videos.

No, it's because I have two little kids.

Human babies need so much care because, well, they come out half cooked.

And these brains are ginormous relative to our body size.

And I'm not just talking about mine, yours too, except for you, Frank.

I'm not sure about you.

But to fit these galaxy-brained noggins through the birth canal, we're born developmentally immature compared to other apes.

I mean, imagine a single mom on the African savannah 200,000 years ago trying to keep a crying helpless infant alive while also foraging for food, not getting eaten by predators, harnessing fire, or whatever else they had to do.

It just doesn't work unless you have a partner to help raise that helpless infant.

Most importantly, a partner who isn't gonna leave.

And this is the insight that completely changed how I understand the biological purpose of love.

Love is a commitment device.

Okay, what does that mean?

Well, if partnering up to raise young was purely 100% rational, there would be this huge risk of defection.

And if both sides of the partnership are constantly calculating whether they can find better genes or better resources elsewhere, neither one invests fully and their offspring don't have as good of a chance.

Love solves this.

From an evolutionary standpoint, romantic love is essentially nature's way of giving you tunnel vision.

Its main job is to kill your wandering eye and broadcast to your partner that you are officially off the market.

This was a huge advantage for our ancestors.

Those who were good at catching feelings and signaling their devotion, they were much more likely to stick together to raise kids.

Because this hopelessly devoted strategy worked so incredibly well for keeping babies alive, the tendency to fall head over heels was passed down the family tree.

And today, it's a nearly universal human experience.

As recently as the 1990s, wait a second, that was like 30 years ago.

Way back in the 1990s, many researchers thought that romantic love was just a cultural thing and it was unique to modern western culture.

Not only was that super wrong, it's also pretty racist too.

Over the past 30 years, researchers have studied hundreds of different cultures and societies and surveyed hundreds of thousands of people from different backgrounds.

And they've found that romantic love is universal.

Essentially, all humans say that they'd rather commit to a long-term relationship with someone that they are in love with.

Now, within those results across cultures, women tend to value romantic love slightly more than men, and the fewer resources someone had or the more children that they had, the more they valued love.

And these support the hypothesis that love evolved specifically to hold partnerships together.

The people who'd be most devastated if the bond broke, we're talking about women carrying the heavier biological cost of reproduction, people with fewer resources to fall back on, parents with more children depending on them, those are exactly the people who rate love as more important.

We see the same patterns of brain activation across cultures too.

Love isn't a western concept or a modern thing.

It's the human brain doing the same thing across hundreds of societies that until recently never had contact with each other.

And that's all thanks to evolution.

But although love does seem to be uniquely human, we can see evidence of similar brain chemistry and behaviors in other animals.

And this might give us some clues to how love evolved in our species.

Meet the adorable prairie vole.

This small North American rodent is special because it forms lifelong pairs bonded to one other individual.

Its close cousin, the meadow vole, is totally promiscuous.

It plays the field literally, I guess, despite being almost genetically identical to the prairie vole.

Now, the difference seems to be that prairie voles have more oxytocin and vasopressin receptors in the reward parts of their brain.

These chemicals flood that reward system when they mate and the partner becomes associated with that reward.

Interestingly, when researchers blocked prairie vole oxytocin receptors, they begin to act more like their meadow vole cousins.

They stopped forming long-term pair bonds.

Long-term love at the molecular level may be as simple as oxytocin and other chemicals hijacking our reward circuits and pointing our brains at one certain individual.

But things don't evolve out of nowhere.

So where did this pair-bonding chemical circuitry actually come from?

The leading hypothesis is that romantic love hijacked the same system that makes mothers and infants bond.

Oxytocin is the same hormone that floods a mother's brain during breastfeeding and caring for their child.

Studies of brain imaging have shown that motherly attachment and romantic attachment, they look a lot alike in our heads.

And does that mean that when you look at your romantic partner, it's like looking at your mom or your baby?

Honestly, that's a question between you and your therapist, but it does suggest that instead of building love from scratch, it seems like evolution took some existing hardware, that powerful bonding mechanism between mothers and they're young, and then repurposed it for adult romantic relationships.

So decades of research have proven that love is a universal human condition and that it's deeply rooted in our biology.

But when you dig a little deeper, there's still a lot about romantic love and partnering up with one mate that still doesn't seem to make sense biologically speaking.

And here, we need to think about game theory.

Game theory is basically the science of how people make decisions when everyone is trying to make the ideal choice for themselves.

What's the smartest move when your outcome depends on what someone else does.

For love, if your goal is just purely to find the most optimal mate, well, shouldn't you always be open to upgrading to a better partner?

Perhaps better resources or better genes, better compatibility?

Well, this is known as the defection problem in evolutionary game theory.

According to that game theory, if everyone else isn't defecting, then well, yes, your right move might be to trade up to someone else, but there's a catch.

If everyone defects, then you get a population where nobody invests deeply in any partnership.

Offspring don't get as much parental care and the whole system gets worse until it collapses.

Evolution came up with some creative ways to keep this from happening.

The first, jealousy.

Jealousy is basically a threat detection system.

It activates when your pair bond is threatened.

Your brain's reward and attachment chemicals have programmed you to protect your bond with that other person.

It's uncomfortable because it's supposed to make you act.

And when we lose that bond, we feel heartbreak.

The grief we feel, it's so painful that it activates some of the same brain regions as real physical pain.

So when a breakup hurts, you're not imagining it.

These are specific things that evolved in our brains to keep us from defecting too easily and crashing this whole system.

Love hurts because according to evolution, it should be hard to break.

And this may solve our paradox of love.

From the individual's perspective, well, love may be irrational, but from the whole species long-term perspective, love is the most rational solution to the problem of sending more of our genes into the future.

There's still plenty we don't understand about love like why do some people fall in love so hard, so fast?

And brain imaging has shown us that the early stage intensity of love, well, that almost always fades and that's natural.

But some long-term couples still show that same activation after decades.

And some people don't seem to be very much interested in romantic love compared to the average human.

All of these may be due to genetic variation in those chemical receptors in the parts of our brain that feel reward and attachment.

But there's plenty of open questions.

Love in the modern world.

Well, it's all pretty different than the world in which love evolved, right?

I mean, for one, romantic love today isn't all about having children and securing resources like it was say a few hundred thousand years ago out on the savannah.

And of course, love can thrive in relationships that aren't just one male and one female.

We are running on hardware that was optimized for a tribe of maybe 100 people in a modern world full of 8 billion people, dating apps, and a multi-billion dollar entertainment industry that tells us what love is supposed to be.

What I mean is, well, I don't think we can blame "The Bachelorette" on evolution.

We do lots of crazy things for love, but maybe a species where people are capable of that level of devotion is a species where everyone does better.

That choice you make for love may be dumb, but it's just an ancient evolutionary program that helped make our species, and that's pretty smart if you ask me.

Stay curious.

Thank you for sticking around to the end of the episode.

We hope you enjoyed this one.

Why don't you head down to the comments and tell me what the dumbest thing that you've ever done for love is.

And I'll go down there and pick my favorites.

I'm definitely not sharing my own.

Now, this show would not exist without the direct support of our Patreon members like these fine folks in our top tier.

Educational content is expensive and time consuming.

We are dedicated to bringing you one-of-a-kind deeply researched science stories.

We think the world would be worse off without shows like this one, and we need your support to keep doing it.

So if you'd like to join our community help us make more videos like this one, check out the link down in the description.

See in the next video.

Love ya.

The Os?

The Os?

I'm incapable of real human speech.