The Selfish Genes That Prospered: The Evolution of Addiction

For the science geeks among you. A lighthearted look at the history and evolution of addiction, including theories explaining why the genes for addiction are so prevalent in modern humans.

Photo of a British Columbian bear from my cousin Julia, a wildlife biologist who has a way cooler life than I do. Who knows; this guy might be going to get some willow bark for his hangover.

Drug Use in the Wild (aka Jane Goodall Attends a Chimpanzee Rave)

Let's start broad: Human beings aren't the only species that takes medicine to alter its internal state.

Grizzly bears, for example, know to rub sore teeth and other injuries against willow trees, whose bark contains salicylic acid - a chemical cousin of acetylsalicylic acid, also known as aspirin.

This is learned behavior, and only some groups of bears have this knowledge.

I love the image of Papa Bear leading his teething son over to a willow tree to demonstrate this hack.

"Not that tree; your cousin Jeffrey rubbed himself against it, and that's why he is the way he is now. Use this one over here - yeah, that's the good stuff."

Most of us have heard about experiments in which rats and other animals become addicted to self-administered alcohol, heroin, and cocaine, but is this something that only happens during manmade experiments? In other words, do wild animals like to get f*cked up?

There have long been reports of animals seeking out intoxicating substances. In the early 19th century, for example, French naturalist Adulphe Delegorgue recounted South African legends about elephants ingesting fermented marula fruit, after which some of them tangled tusks while others collapsed into a stupor.

In the Caribbean, teenaged vervet monkeys know how to imbibe the fermented sugar cane juice produced on nearby plantations. They also like to steal alcoholic drinks from bars.

When monkeys get drunk, their staggering, sloppy behavior is eerily similar to what you'd see in any human bar 45 minutes before last call. In an episode of Weird Nature, observers noted that - just as in human populations - some monkeys drank nothing, most drank a little, and a few drank to excess.

Try as I might, I can't find any literature on whether these monkeys and elephants have knowledge of hangover remedies, but the image of a hungover vervet monkey stumbling over to a white willow for some relief is almost too good to be true.

It turns out that many species appreciate a psychoactive break from reality: Cats tweaking on catnip, dolphins cultivating a pins-and-needles high by eating pufferfish, lemurs coating themselves in secretions from toxic millipedes, which not only repel insects, but also have a narcotic effect.

It's not just mammals that have a predilection for psychoactive substance use, either: Some bees reportedly prefer nectar containing nicotine and caffeine. It's sort of like Addy for insects, I guess.

Speckled Moths and Sickled Cells: A Quick Recap of Darwin's Theory of Evolution

*If you have a basic understanding of Darwin's theory of evolution by natural selection, feel free to skip this section.

One of the crucial components of Charles Darwin's theory of evolution is that natural selection roots out genes that decrease an organism's fitness*.

*Fitness refers to an organism's capacity to survive and reproduce.

For a quintessential example, let's take the Peppered Moth, which was widespread in the UK during the nineteenth century. Most of these moths were white with black speckles, which allowed them to camouflage themselves against the lichen that they liked to land on (see pic).

Peppered Moth white phenotype. Before the Industrial Revolution, this was the most common phenotype. Photo from butterflyconservation.org.

Although most of the moths had white wings with black freckles, there was a mutation that caused some moths to produce more melanin, which resulted in wings that were almost entirely black. However, this all-black phenotype tended to stand out against the foliage that the Peppered Moths lived in, and for this reason, black-variant individuals tended to be eaten by predators. Thus, the black-variant moths had more trouble surviving and reproducing, so they became less prevalent in the population over time.

This was all true prior to the Industrial Revolution. In the 19th century, however, pollution from industrialization and domestic coal fires changed the moth's habitat dramatically.

Against this new backdrop, it was the black-variant moths who were able to hide within the soot-covered foliage (see pic). This caused the white-winged moths to virtually disappear. By 1895, for example, 98% of the Peppered Moths in the city of Manchester were black.

Black phenotype of the Peppered Moth (left). Unfortunately, the best photo I could find was from the Institute for Creation Research, which is one of those anti-evolution Christian groups. But hey, they take (or steal) a good picture, I guess.

This case study illustrates the important point that fitness is specific to a given time and place. As an organism's environment changes, traits that were once favored by natural selection can become maladaptive.

Evolution by natural selection isn't always so straightforward. For instance, take the gene for sickle-cell anemia, a human disease that changes the shape of red blood cells. Rather than their normal, disc-shaped form, affected red blood cells collapse into a sickled shape (see pic).

Normal red blood cells (disc-shaped) versus sickled ones (collapsed; spindly). I miss med school. Photo from MIT News.

These sickled cells do not carry oxygen as effectively. Moreover, they tend to stick together, which leads to blockages that cause painful, damaging interruptions to blood supply.

The malformed cells can also accumulate in the spleen. These so-called sickle-cell crises lead to swelling, scarring, and a loss of the oxygen-carrying protein hemoglobin, which can be life-threatening.

Why didn't evolution by natural selection wipe out the gene for sickle-cell anemia? Obviously affected individuals would be less likely to survive and reproduce, right? So, over time, evolutionary theory would seem to predict the disappearance of sickle-cell anemia from the human population.

Scientists used two key clues to solve this mystery. The first was who gets sickle-cell anemia: It's a disease that predominantly affects people descended from sub-Saharan African populations.

The second clue was found in the mechanism by which sickle-cell anemia is inherited. It's an autosomal disease, which means that it is not sex-linked (found on the X or Y chromosome). It is also a recessive disorder, meaning that only individuals who have two copies of the mutated gene - one from mom and one from dad - will have the disease.

With a recessive disorder, individuals may carry one copy of the affected gene without themselves manifesting the disease. Although carriers of the sickle-cell gene do not have symptoms of sickle-cell anemia, they do make some malformed red blood cells (just not enough to cause sickle-cell crises).

These carriers can, however, pass the mutated gene down to their offspring. Again, if their children inherit another mutated gene from the second parent, then they will have sickle-cell anemia.

Putting these two observations together with one final piece of information, scientists made a brilliant deduction.

For much of the history of sub-Saharan Africa, malaria has been one of the leading causes of death. The malarial parasite lives in red blood cells; infection is spread through the bite of mosquitoes.

What scientists realized was that the malarial parasite doesn't like sickled red blood cells; in fact, affected individuals seldom get malaria.

Moreover, even carriers of the sickle-cell gene are protected from the parasite. It seems that they make enough sickled red blood cells to be protected from infection, but not enough to manifest sickle-cell disease.

So, the going theory is that the sickle-cell mutation persisted in the sub-Saharan African population because the protection from malaria conferred on carriers and sickle-cell anemics outweighed the deleterious effects of the sickle-cell disease on those who were affected by it.

Sickle-cell anemia demonstrates another important point: That genes that seem to be maladaptive in one way might be beneficial in another.

So, to recap, Darwin theorized that inherited traits are acted on by natural selection, which favors traits that increase organisms' ability to survive and reproduce. But fitness is context-specific, and genes can be inherited in several modes. That is where things can get tricky.

Is Addiction Inherited?

It's hard to imagine a behavioral trait more detrimental than addiction. Addiction is dangerous and distracting; it causes harm in myriad ways. It blinds us to significant risks by inducing artificial euphoria that banishes pain and anxiety.

From the point-of-view of natural selection, it's particularly relevant that addiction decreases reproductive fitness. Regular use of substances can decrease fertility in both men and women, it can increase the chance of miscarriage and damage to the baby, and it certainly makes both men and women less capable parents than they would otherwise be. These changes drastically decrease Darwinian fitness.

Why, then, is addiction so prevalent? Shouldn't natural selection have taken these "selfish" genes out?

The first question to ask is whether addiction is really inheritable - in other words, whether differences in individuals' DNA that predispose them to addiction are passed down through families.

After all, it's easy to understand how children of addicts could become addicts themselves just by emulating the behavior of their dysfunctional parents. It's not necessary to have a genetic component to explain why addiction runs in families.

Scientists have clever ways to parse the available data to answer this question. One method is using twin studies, which compare rates of a disease in identical twins separated at birth. In such cases, the DNA is identical, but the environments are totally different. If both twins end up developing addiction despite different environments, and this happens frequently across many pairs of identical twins*, it suggests that addiction has a genetic component.

*And less frequently in fraternal twins, who share 50% of their DNA.

The scientific consensus is that there is a powerful genetic component to addiction. In other words, addiction is passed down in families not just because of shared environment and learned behaviors, but also because of genetic differences that influence whether an individual develops addiction or not. As I have written about elsewhere, this is true of Cluster B personality disorders as well as many other mental illnesses and complex behavioral traits

Scientists and statisticians use a variable called R, which they refer to as heritability, to describe how much of the variation in a trait within a population is due to genetics. For addiction to various drugs, values between 0.4 and 0.8 have been given for R, meaning that about 40% to 80% of the risk of developing addiction is down to genes. (For those of you who want to consult the literature, Cambridge Core has a great review of the genetics of substance use disorders for various drugs here).

To summarize, the robust data on this topic show that addiction is in fact inheritable and that 50% or more of one's risk of developing addiction is due to genetic differences*.

*This is not the most precise interpretation of R because R describes population variation, not individual risk, but because it is the most easily digested and applied formulation, it's sometimes used even by scientists and statisticians.

Addiction is a complex behavioral trait that is influenced by multiple genes, so the genetic picture is more complicated than it is for a binary, single-gene trait like wing color in Peppered Moths. Interestingly, some genes that have been associated with addiction are specific to certain substances, and several of them are correlated with the risk of other mental illnesses, too.

Quick note: For these next sections having to do with specific theories on why the genes for addiction are so prevalent in modern human populations, I originally wrote one screed, which was unreadably long. I then separated my points into sections, but because they overlap, it's more a case of variations on a theme then entirely self-contained arguments. I stuck to a different line of evidence and angle of approach for each, though.

(1) Evolution Takes Time

We were able to observe evolution by natural selection so clearly in the Peppered Moths partly because they reproduce so quickly. Peppered Moths emerge from their cocoons, mature, and lay eggs within the space of 4 to 7 days. Their eggs then take 4 to 10 days to hatch, meaning that the full generation time from birth to producing offspring is 8 to 17 days.

The Peppered Moth's generation time is so short that hundreds of generations passed by during the first couple of decades of the Industrial Revolution. We were able to see evolution occur so quickly for this reason and also because the selection pressure was so strong: White moths stood out so markedly against the soot-stained backdrop that predators quickly eliminated them, causing evolution at an accelerated pace.

In humans, by contrast, evolution typically takes tens of thousands to millions of years. Our species has an average generation time of 25-30 years*, Moreover, for a complex trait like addiction that is tied to many emotional and behavioral traits, natural selection is likely to be slower and gentler.

*There is a very interesting article about the average generation time of humans over the past 250,000 years available from the NIH's National Library of Medicine here.

For this reason - i.e., that human evolution usually takes tens of thousands to millions of years - most mainstream biologists treat the claims of popular "evolutionary psychologists" with skepticism. Evolution has shaped our capacities for language and for various forms of social behavior, but it can't explain highly specific behavioral responses to modern environments.

Even if the genes for addiction are being weeded out by natural selection, evolution hasn't had enough time to have a strong impact yet. There just hasn't been enough time elapsed.

(2) Availability of Substances

Addiction in its intensely detrimental modern form hasn't been around for that long in the grand scheme*, either.

*Remembering that our species first appeared about 300,000 years ago.

However, archaeological evidence indicates that early hominid species were using psychotropic drugs as far back as 200,000 years ago. Nicotine from tobacco, opium from poppy plants, cannabis, cocaine from the coca plant - multiple lines of evidence indicate tens of thousands of years of psychotropic substance use all over the world.

However, for the vast majority of human history, substance use was naturally limited by availability and the need to dedicate most of one's time and energy to obtaining food, shelter, and other basic necessities. For this reason, ancient civilizations commonly treated these psychotropic substances as special, and there was often a spiritual quality imputed to their experiences with them.

It wasn't until the past couple of centuries that we were able to isolate and mass produce the mind-altering substances found in these plants. Techniques to chemically alter these natural substances to produce more potent synthetic ones have only been around for a few decades.

So, although ancient humans likely chewed on coca leaves to boost productivity or smoked cannabis to get closer to God (see pic), it wasn't until the modern era that powerful, mass-produced substances were continuously available. Without this condition being met, addiction was a nonissue.

I typed "a picture of a happy caveman smoking cannabis by a fire" into Gencraft's free AI image generator and this was the (pretty based, I must say) result.

(3) Substance Use Was a Clever Happiness Hack

During the tens of thousands of years before modern chemistry made mass-produced mind-altering substances available, ancient peoples used plants like cannabis and coca seeds to relieve pain, boost energy, and enter states deemed more conducive to spiritual experience.

Addiction is rooted in the midbrain, which releases feel-good neurotransmitters like dopamine to reward behaviors that help us to survive and reproduce. So, when we eat calorie-rich foods and have sex, for example, we are rewarded with dopamine hits that give us a rush of contentment.

Addiction hijacks this system by artificially stimulating these same dopamine-releasing systems, which delivers euphoria that is often much stronger than naturally occurring responses (see chart below).

I've seen values of 500 to 750% for opioids. Porn can crank things up to 200 to 250%, so Jenna Jameson on a computer screen might beat the girl next door in 3-D, unfortunately.

For most of human history, life has been short and brutish. People were hungry most of the time, and even when they weren't, they suffered from nutritional deficiencies because there wasn't enough variety in their diets. Pregnancy was just as likely to lead to death of the mother and / or miscarriage as it was to a live, healthy birth, and many of the children who made it into the world perished during childhood due to rampant infectious diseases.

Something as simple as a rotten tooth could lead to agony and death; skeletal remains of early humans show extensive damage from infection, cancer, and chronic diseases. Early peoples put tremendous effort into building self-sufficient settlements only to have famine, disease, or war decimate them.

In this dire context, people whose brains rewarded them for using mind-altering substances actually might have had a better chance of surviving and reproducing. Using the relatively mild, plant-based substances available for the first tens of thousands of years of our species' evolution allowed them to cushion themselves from the harsh realities of the world that they inhabited.

From this point-of-view, substance use was an adaptation (and a pretty common one, judging by the co-evolution of human brains and plants that make mind-altering substances through the millennia).

Positive psychology has taught us that happy people are more productive, mentally and physically healthy, and resilient. If taking drugs boosted your mental state and this helped you to cope with a fraught environment, then it would have increased your Darwinian fitness, at least at first glance.

As mentioned above, it wasn't until the past couple of centuries that potent, highly addictive drugs became widely and regularly available. This was when substance use took a turn, fitness-wise.

During the Industrial Revolution in England, when there were mass migrations of people from the country to the cities to work in the factories and other enterprises that sprung up, alcoholism was so prevalent that some factory owners gave their employees Mondays off to recover from the post-weekend shakes.

Opium use was another popular pastime, and it often involved imbibing a combination of alcohol and opium known as laudanum. The list of writers, artists, and other famous figures who were opium addicts is staggeringly long; substance use is thought to have played a role in the death of Edgar Allan Poe and other literary legends.

Preachers ranted against the evils of intoxicating substances while Op/Ed pieces dissected the roots of and potential solutions to the problem. An 1887 newspaper column called "The Opium Habit - The Most Abject of Slaveries - Is There Any Emancipation?" provided a lively depiction of the plight of the addict: "[they are] hopeless, helpless slaves, mind weakened, lacking energy for any effort toward recovery, rapidly drifting into imbecility and untimely graves. A peculiar feature is that victims craftily conceal it from their nearest friends" (see pic).

*Screenshot from the Library of Congress' digital copy of the Southern Standard paper of McMinnville, Tennessee, dated November 19, 1887. The same publication that railed against opium addiction touted a "health-preserving, pure and wholesome" Safe Yeast cure - God knows what that had in it.

I have written elsewhere about my belief that the long-term mental, physical, and social effects of opioid maintenance medications like buprenorphine and methadone will parallel those of the opiate wasting syndrome recognized as far back as the Victorian era. By that time, there was a recognized syndrome of physical and cognitive decline in those dependent on opium and morphine, and it was thought that they were at increased risk of GI cancer and other ailments, as well.

Thus, addiction in its highly detrimental form is a modern phenomenon. Before that, for most of our species' development, natural selection might actually have favored the genes that predisposed individuals to enjoy mind-altering substances because it helped them to maintain a positive emotional state when contending with a challenging environment.

(4) Mind-Altering Substances Might Even Get You Laid

Perhaps the effect of mind-altering substances on mating was significant, as well. As comical as it is to imagine a group of early humans chewing coca leaves by the handful to rev themselves up for a prehistoric orgy, the idea has some merit.

Humans are a complex, highly social species with nuanced, variable mating rituals. And, as any incel will tell you, competition for mates is intense. If mind-altering substances decreased the mental barriers to mating by alleviating anxiety and inhibitions, it's possible that genes favoring their use would be selected for over the hundreds of thousands of years of our species' development.

In layman's terms, getting f*cked up made it easier to get f*cked, and that's a Darwinian win.

(5) Addiction as Collateral Damage

As we saw in the case of sickle-cell anemia, sometimes a gene for a deleterious trait such as sickle-cell anemia can persist in the population because it confers a strong benefit, such as protection from malaria.

I touched on the idea that addicts have particularly adaptive dopaminergic reward systems that are naturally at a low level of stimulation above. As I mentioned, this system evolved to reinforce behaviors that helped us to survive and reproduce - for example, eating high-calorie foods and having sex.

Finding food was difficult for our atavistic ancestors, who often spent months and years in near-starvation. Mating certainly wasn't easy, either. Our primate ancestry means that the males of our species are in intense, sometimes violent competition for desirable females, and God knows that being a human female isn't easy, either.

Thus, nature served up a nice reward for killing that deer for dinner or scoring that hot, knuckle-dragging biddy by the fire. Individuals whose brains rewarded them strongly for such behavior were more likely to repeat it, and addicts' brains certainly seem to crave - and to come to depend upon - these dopaminergic hits more quickly and strongly than other individuals'.

The problem is that this dopaminergic system also reinforces other behaviors, including gambling and drug-taking.

Now, this wasn't such a problem in ye days of olde. Our ancestors were too busy trying to survive to squander fortunes in casinos or to smoke weed all day.

It's only modern society that has served up enough vice to allow compulsive gambling and drug-taking to become life-threatening problems. Similarly, although the midbrain system did evolve to reinforce sex and eating, it's only in modern environments that nymphomania and overeating have become real threats to health.

Before this, the addictive brain evolved because it was more sensitive to the midbrain dopamine reward system, which made addicts more motivated to survive and reproduce than other individuals. The fact that this system has been hijacked by drugs in modern times is just unfortunate collateral damage.

(6) Addiction is Tied to Adaptive Behavioral Traits

I've written elsewhere about Cluster B personality disorders, which include Narcissistic Personality Disorder and Borderline Personality Disorder. People with these disorders are demonized; they are treated as a scourge on modern society.

Addicts share some psychological characteristics and behavioral traits with sufferers of these disorders. Traits common in addicts, including impulsivity, emotional volatility, high anxiety, and selfishness, seem totally maladaptive from the contemporary perspective.

The thing is that they only seem maladaptive because we take our current social context for granted. It's only during the past several hundred years - the blink of an eye from an evolutionary perspective - that life has been peaceful enough and society complex enough that we began to place great emphasis upon "prosocial" behaviors such as altruism*. kindness, and nonviolence.

*The existence of true altruism in any species is hotly debated by evolutionary biologists.

Before then, life was a rough-and-ready affair. When the plague hit or a rival clan showed up with spears and clubs, addicts were the individuals who responded most rapidly and energetically. Whether we fought or fled, our impulsive, anxiety-prone brains helped us to react quickly and decisively. Our selfishness and our tendency to self-isolate allowed us, and likely our relatives, to escape dangerous situations.

So, one way to conceptualize addiction is that compulsive drug-taking is a negative behavioral trait that is tied to a cluster of personality and behavioral qualities - including impulsivity, emotional reactivity, anxiety, and selfishness - that served our ancestors well for much of our history.

At the molecular level, these differences come partly from variations in dopamine receptors. Different people have variations on the gene that specifies how to make this receptor, and because of this, our midbrain reward system can be more or less reactive (less reactive dopamine receptors mean that individuals need more stimulation to trigger the reinforcing hit).

Intriguingly, human migration patterns correlate with dopamine receptor genetics. People whose midbrains demanded change and excitement were more likely to migrate across the Bering Strait crossing, then move all the way to South America. "Calmer" people likely stayed put in Eurasia.

These biological differences helped to create culture - for example, consider the value placed on self-regulation, fitting in, and being nonreactive in Asian cultures versus the North American emphasis on vigorous self-defense and florid self-expression.

It's possible that addiction is such a problem in countries like the United States and Australia because we are essentially frontier societies. We attracted the people who were restless, independent, impulsive. Our ancestors were the individuals who - after a night of drinking and talking s*it about getting out of dodge - actually woke up the next morning and signed themselves into indentured servitude so that they could make it to the New World.

Some of our predecessors were criminals or religious outcasts; they had the courage to thumb their noses at polite society in their home countries and take a big gamble on geographical change. Again, all of these traits were probably useful for the majority of human history and before that, the evolution of the early hominids that we are descended from.

The fact is that the kinds of brains predisposed to addiction are also the kinds of brains that were most likely to survive during our tumultuous past.

Final Thoughts - You Seriously Read This Far?

The genes for addiction evolved because they were correlated with adaptive behavioral traits during the hundreds of thousands of years before modern society made it possible to squat in an abandoned house shooting dope with dirty needles all day.

Addicts have a restless, impulsive, sometimes antisocial brain chemistry. This made us and our relatives more likely to survive the war, famine, and disease that characterized much of our ancient history.

Understanding this provides insight into why the rates of addiction vary across different societies. Analyzing differences in brain chemistry, particularly those related to the midbrain dopaminergic reward system, helps us to untangle the complex web of hereditary and environmental factors that influence risk of addiction.

One worrying trend is the demonization and criminalization of certain brain chemistry profiles, such as those of addicts and Cluster B personality disorder sufferers.

It's no coincidence that so many famous figures suffer from these and other mental disorders. The same restlessness and emotionality that cause problems in our social function also allow us to produce gut-punchingly vivid art. These differences also give us the social separation necessary for effective societal critique.

Even Machiavellian traits like manipulation and power-hunger can be adaptive if they are harnessed properly. Dark Triad traits - psychopathy, Machiavellianism, and narcissism - are more common in middle and upper management than they are in the general population, and it's thought that this is true because these traits aren't always maladaptive in these settings. When you're a CEO faced with tough budget cuts, being able to emotionally separate yourself from the people affected by your layoff decisions is not necessarily a bad thing.

By understanding the origins, strengths, and limitations of different brain chemistry profiles, we can use them to maximum positive effect at both the individual and societal levels. There is absolutely no shame in having any particular mental illness or personality profile. It's not what you're predisposed to that counts; it's what you actually do with the cards that you're dealt.

It's possible that if modern society stays roughly the same for tens of thousands of years, we will see weeding out of the genes for addiction from the population (especially with ultra-potent analogs like fentanyl and carfentanil killing people who are predisposed to experimenting with drugs early on in their lives).

Again, however, this isn't necessarily a good thing. Were our environment to suddenly shift due to climate change, conflict, or some other catastrophe, those same "negative" traits seen in addictive brains might suddenly be the ones that save us, after all.

Thank you for reading! I have a whole "science of addiction" series planned, but because these posts take longer to put together, I've been putting them off until the summer months.