This episode of SciShow is sponsored by Brilliant.

Go to Brilliant.com/SciShow to learn more. {♫Intro♫}. There are all kinds of anecdotal cases of animals getting stoned or tipsy—like, the ones we talked about in our list show of high animals.

But as we noted for the dolphins getting high on pufferfish or the elephants having a nightcap of marula fruit, pretty much every case we’ve examined closely has found that there isn’t any evidence that these animals consume mind-altering substances because they feel good to them. Which ultimately makes it appear that the fondness for psychoactive drugs is, pretty...human. And it raises questions about why.

What is it about humans that makes us so attracted to these substances? Totally upfront here, we don’t have a solid answer to that question. If that’s what you’re looking for, you can go watch videos where they say things without knowing them for sure.

The internet’s full of them. You might think we do. You might have heard that these drugs hijack our mental reward systems, so obviously, we seek them out because our brains have this inherent weakness for them.

But some scientists have pointed out that that whole idea doesn’t hold up as well as you might think. And by digging deeper into how our closest relatives and our laboratory proxies interact with mind-altering substances, we can get a better understanding of what might really be setting us apart. The simple answer to why humans like drugs appears to be quite obvious: they feel good.

In neurological terms, that means that they activate the parts of our brain that signal rewarding experiences. And this seems pretty clear when you look at drug use in lab animals. Mice and rats are used to model many human illnesses, so it makes sense that they’ve been used to study why humans seek out drugs, too.

And we know that they can become addicted to almost anything: nicotine, alcohol, cocaine… you name it. Left to their own devices, rodents will often self-administer drugs until they become very intoxicated or even overdose. This has allowed scientists to study the genetic basis and brain chemistry of drug-seeking behaviors, as well as test ways of treating addictions.

And such research seems to support the idea that drugs are just really good at hijacking mammalian brains. But these studies don’t actually confirm that non-human animals would do drugs under normal circumstances, or that they are something the animals enjoy. You see, the whole setup is pretty artificial.

In many cases, researchers use what are called self-administration models, where the animals end up at least somewhat controlling how much of the drug they get. But to convince lab animals to take the drugs in the first place, researchers do all kinds of strange things—like starve them. That’s all well and good if you want to get a rat hooked on something so you can see what happens to their body or brain when they are.

But they aren’t seeking that drug because they like it—they’re motivated by a real biological need like hunger. Or the animals may be put in a situation where they’re so bored that doing anything seems better than nothing. And that might be why all you have to do to disrupt their drug-seeking behavior is to make their environment more ecologically realistic.

Lab animals often reduce their drug use or even lose their addiction when you make their lives more interesting—like, if you give them friends to hang out with or place toys or running wheels in their cages. If these animals were truly seeking the drugs for the feeling of being high, or the drug’s effects were powerfully rewarding on a neurological level, it shouldn’t be so easy for them to just say no. Of course, as we often say here on SciShow, mice and rats aren’t people.

Neither are apes and monkeys, but they are at least primates and therefore a lot more closely related to us, so they might give us more insight into why humans seek out highs. And there are cases of other primates that choose to intoxicate themselves—like, on the island of Saint Kitts in the Caribbean. There, vervet monkeys are infamous for stealing tourists’ cocktails almost right out of their hands.

This has become a bit of a joke, but the monkeys actually inspired a series of scientific studies beginning in the late 1980s. One found that seventeen percent of Caribbean monkeys brought into the lab preferred a mix of alcohol and sugar water over a drink of just sugar water. And it seemed like the drunk monkeys were behaving pretty much like drunk people—falling off of perches, getting cozy with other monkeys, or just sitting alone with a bottle in the corner.

But, we can’t really tell from early experiments like this what’s motivating the monkeys to drink. While they eagerly consumed booze, it seems unlikely they were seeking out alcohol for its intoxicating effects. Some scientists think that, instead, they may have fallen into an evolutionary trap.

That’s where a trait that was useful for the survival of a species is now harmful instead. See, the taste and smell of alcohol may be similar to ripe fruit, which is more likely to contain a lot of sugar and, therefore, calories. And when there’s lots of competition from other fruit-eaters, and relatively little fruit, those monkeys want to make sure they get as many calories as they can per bite to ensure they get the nutrients they need.

So the monkeys may have thought the presence of alcohol indicated a high-quality food. In fact, when the Saint Kitts monkeys were first brought to the island around three hundred years ago, they were often seen feasting on sugar cane, particularly the fermenting kind that contained alcohol. Seeking out that tasty, sweet treat might have later led to going for drinks that smelled or tasted similar, like a tourist’s fruity cocktail.

But, the cocktails aren’t great for the monkeys nutritionally-speaking. Also, that is true for the tourists. But getting drunk puts the monkeys in risk of harm in all sorts of ways.

So an evolved attraction to booze that was meant to help them survive may have turned on them when stronger doses of alcohol became available. Then, they simply weren’t able to learn that the booze is harmful. But, it’s also important to note that only 17% of those monkeys preferred the alcoholic beverage.

So even if an evolutionary trap explains their preference, it’s not a trap all of them are falling into. There are other primates that consume alcohol which we can consider—like the group of chimpanzees in Guinea that regularly steals fermenting raffia palm sap. But the chimps in question don’t really help us figure out if primates are prone to this kind of evolutionary trap, because they live in an area where people have all but destroyed their habitat and natural food sources.

If humans weren’t destroying these animal’s homes and providing concentrated alcohol, the animals might not go for the calorie-laden liquid. In fact, in lab studies, most chimps prefer non-alcoholic fruits over fermented ones, which isn’t what you would expect if drinking was something universally enjoyable to them or they had evolved to associate alcohol with better food sources. Now, it’s possible humans have fallen into an evolutionary trap of some kind when it comes to psychoactive drugs.

But the prevailing hypothesis isn’t that they were once a signal of something good, and have since become bad for us—it’s that they trick our brains’ into making us think they are awesome for us, even though they aren’t and never have been. The basic idea is that the neurological pathways we use to evaluate things that are good for us developed in the absence of psychoactive substances. You see, our brains’ reward system is supposed to identify things we need to survive—like food—and when it activates, it can either block or overwhelm feelings that tell us something is bad, like pain.

It’s possible drugs just so happen to trigger this system even though they aren’t something we need to survive. And if, evolutionarily speaking, they’re a new thing, then we wouldn’t have had time to evolve countermeasures so that we don’t get fooled. But that idea doesn’t hold up as well as you might think.

It’s not clear when this magical time was where these substances weren’t around, for example. The entire lineage of humans—the hominins—evolved enzymes to detoxify alcohol some ten million years ago. It would be pretty weird for that trait to have just evolved if we weren’t regularly being exposed to booze.

And the main group of enzymes that we use to neutralize plant toxins date back even further. Cytochrome P450 haemoproteins have been detoxifying the compounds found in plants like tobacco and coca for us and our other animal kin for about four hundred million years. And, really, it doesn’t make sense that these chemicals would accidentally affect our brains so positively anyway.

Plants produce things like nicotine, morphine, and cocaine to be toxic and stop us and other plant-eaters from consuming them. Some scientists have pointed out that it doesn’t really make sense for toxins meant to fend off creatures like us to accidentally encourage significant consumption instead. Scientists refer to this evolutionary mismatch as the paradox of drug reward.

One intriguing explanation for this paradox is that the triggering of reward systems isn’t accidental at all—we’ve actually evolved to find them rewarding because they have medicinal value. See, popular drugs like caffeine or nicotine do what the plant evolved them to do: they harm animals. And there are cases where we want to harm animals inside us—specifically, parasites.

And we see evidence for this idea in human studies. For example, a study of the Aka, Indigenous people of the Congo Basin, found that men who smoked more tobacco had a lower parasite burden, and they were less likely to be reinfected with a parasite one year later. Many other psychoactive drugs also have antiparasitic effects.

If eating these plants helped us survive in the past by getting rid of nasties inside us, our brains could have evolved to reward us for eating them. Or, our affinity for mind-altering substances could somehow stem from our minds. Basically, some researchers suggest that there are complex social or psychological benefits to psychoactive substances and that those are what drive our attraction to them.

So, it’s not that the drugs “hijack” our brains… it’s that our brains exploit drugs to maximize their benefits and downplay their costs. Alcohol lowers your inhibitions, for example. That’s why people call it “liquid courage”.

And if, say, you became more talkative at a party the first time you drank alcohol, then even if you felt physically terrible, the reward you got from the social behavior might have made the overall experience a positive one. There could also be psychological benefits to drugs. Like, cocaine might give you a temporary cognitive boost.

If the reasons we find drugs rewarding are social or psychological, then it wouldn’t be so surprising if they were unique to humans. After all, our socialness and cognitive abilities are somewhat extreme when compared with our fellow animals. And we might be uniquely capable of learning how to use psychoactive substances to our advantage, and not to our detriment.

But that’s a hard hypothesis to test, in part because it’s just really hard to study the motivations of animals that can’t tell you what they’re feeling or thinking. The bulk of research on animals and drugs is to understand the mechanisms of addiction. Which makes sense, because it’s a big public health problem.

And to be clear, none of these hypotheses about why we started using psychoactive substances really speak to that side of drugs. The neurological basis of addiction—like, that the over-use of substances which trigger the reward system can lead to a chemical dependence on them—holds whether that reward is an accident of evolution or not. Which is why models like rats and mice are still useful because if you want to study the biology of addiction, it doesn’t really matter how the animals get hooked, just that they are.

But if you want to know if there are other animals that truly enjoy being high, rodents can’t tell you much. Even studying non-human primates has ultimately left us with more questions than answers. In the end, we don’t really know why our species is so enamored with mind-altering things.

But the more we look into it, the more we learn about our complex relationship with psychoactive substances—and the closer we get to some real answers. It’s going to take a lot of research and problem-solving to get to the bottom of this. And if that sounds like your cup of tea, you might want to check Brilliant’s Daily Challenges.

Every day, Brilliant releases new challenge questions that cover fields from computer science to statistics. Recently, for example, they had one about how to win at Twenty Questions. Which is my kind of challenge.

You can access the Daily Challenges for free, but if you sign up to become a Premium member, you’ll get access to the whole archive. So whether you’re chilling on the bus home from or just want to brush up on your skills, you’ll have plenty of content. Also, if you really like a challenge and want to learn more about the subject, there’s a related course that explores the same concept in greater detail.

You can learn more about Brilliant.org/SciShow. And if you’re one of the first two hundred people to sign up there, you’ll get twenty percent off an annual premium subscription. {♫Outro♫}.