Why Legalizing Marijuana Leads to a DROP in Teen Drug Use (hint: evolution)

As of January 2018, twenty-nine U.S. states have legalized the use of marijuana for medical use. Another fourteen have decriminalized possession of the drug, and eight states have fully legalized marijuana even for recreational use. Critics of this trend, including Attorney General Jeff Sessions, have long argued that legalization of marijuana “will lead to more marijuana use, including among children and teens, causing all sorts of public health problems down the line.”

In short, those favoring strict drug laws believe that, as marijuana becomes more available and less stigmatized, teen drug use will go up. It’s a straightforward and logical belief. The problem is that it’s wrong.

The reality is that, to date, not one jurisdiction, either in the U.S. or elsewhere, has seen a marked increase in teen drug use following the relaxation of marijuana restrictions. Not one. Both Colorado and Washington, the pioneer states of marijuana legalization, have actually seen drops in teen marijuana use following legalization. The drop in Colorado was particularly dramatic. Despite the wave of marijuana legalization, nationwide, teen drug use is at a 20-year low.

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This is no quirk, nor is it just too early to see the inevitable spike in teen drug use. The spike was never going to happen. U.S. drug policy is predicated on the seemingly straightforward belief that increased access leads to increased use and that increased enforcement leads to decreased use. Simple, logical, but wrong. In fact, as far as teens and young adults are concerned, that logic may be exactly backwards.

Access and consequences are just two factors that influence a teenager’s decision to use drugs or not. Obviously, if access is completely restricted, then there is no room for a “decision” to be made in the first place. However, since the nearly five-decades long “war on drugs” has failed to remove access from the equation, we have attempted, with the best of intentions, to stigmatize drug use instead. We educate our children about the health and personal dangers of drugs and the harsh penalties for getting caught. Unfortunately, this approach fails to consider how adolescents actually make decisions.

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Although evolutionary biology may seem like a strange field to look for guidance on drug policy, the phenomenon known to biologists as “costly signaling” explains why stigmatization is altogether ineffective at reducing drug use among teenagers. In many species, including humans, individuals engage in various behaviors meant to signal to others that they are healthy and strong, a desirable mate and a formidable rival. As I describe in my new book, “Human Errors,” these costly signals can take many forms, but among them are behaviors that are seen as risky, dangerous, unhealthy, and even foolhardy.

The logic of the coded message is simple: An individual must be impressive if they can withstand the costs and dangers that come with risky behaviors. Costly signaling is believed to be a major reason why risky behaviors are so much more prevalent among rapidly maturing teenagers, especially males, than they are in children or adults. Among many social animals, males are known to incur great costs and risks in order to advertise their strength and vitality to peers, and these costly signals are usually effective.

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Like many evolutionary behaviors, fitness displays are not done consciously; individuals don’t intend for these actions to serve as advertisements to others. But there is good evidence that this is what drives the adolescent impulse toward risky behaviors. For instance, unless they have developed a dependency, teenagers rarely partake in drugs or alcohol when they are alone. This makes sense from an evolutionary perspective. Costly signaling only works when there is an audience of peers.

Most drug deterrence initiatives repeat the refrain that drugs are harmful and taboo. But this is precisely what makes them so attractive to teenagers, especially teenage boys.  In their minds, it frames drug use as an opportunity to show off to others and advertise fitness. The greater the stigma against marijuana, the more valuable the costly signaling is for teenagers who dare to buck the taboo.

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The opposite is also true. When marijuana is legalized and society acknowledges that the drug can be enjoyed safely, smoking pot is no longer a costly signal. Of course, this does not deflate all interest in the drug among teenagers, but it does undercut the stigma of danger that is so attractive to teenagers desperate to prove themselves to their peers. When marijuana use is legal and safe, decision-making about it among teenagers follows a more predictable, adult-like pattern. No one argues that legalization will solve all of our drug problems – adults get hooked on drugs also – but it may indeed do more good than harm, at least for marijuana use among teenagers.

There are many factors to consider as we debate drug policy, but the claim that criminalization and stigmatization will drive down teen drug use flies in the face of basic evolutionary psychology. In this and other areas of public policy, the emerging message from decades of social science research is that easing taboos on dangerous behaviors actually helps our most vulnerable citizens. Unfortunately, under the new leadership of the Department of Justice, drug policy appears to be taking shape despite the scientific evidence, rather than because of it.



15 thoughts on “Why Legalizing Marijuana Leads to a DROP in Teen Drug Use (hint: evolution)

  1. Despite scientific evidence… Thank you for speaking out about this. We also see this denial of science-based research in Scott Pruitt’s push back on regulating pesticides and the Department of Energy and the Pentagons desire to bring back nuclear arms. It is not the health of individuals that concerns many politicians, it’s their control over policy. Arresting youth of color for possessing pot was a political maneuver to keep control over a certain population. Unfortunately, the driving force behind decriminalization was so that states could have a new revenue stream, not because politicians were concerned about our youth (Oxycontin is another example of this – lobbying by the pharmaceutical industry ($) despite the evidence of it’s abuse).
    Good to hear that evolutionary biology plays a role in the fact that fewer youth will be smoking pot and that fewer youth will be incarcerated. No doubt that science-based evidence and policy rarely overlap, so we’ll slug along on our evolutionary adventure until we hit a tipping point and then maybe – just maybe- we’ll get smart.

    Liked by 1 person

    1. Agreed on all points. Pursing an idealogical political agenda is one thing, but when the evidence refutes the stated goal, but they pursue it anyway, then clearly the true motives are different than the stated ones, as you rightly point out.


  2. Really interesting point. Not so surprising for the ones who remember what made smoking a joint so attractive. But could there be a second order effect that pushes teenagers to experience with other illegal drugs (e.g. cocaine) to keep signalling? Or to engage in other dangerous activities (extreme sport)? In a very twisted way, it could be argued that keeping cannabis illegal is a way to allow signalling for teenagers in a relatively harmless way.

    Also it seems that pot legalization increases use for adults but might decrease use of other drugs, notably alcohol (probably a net benefit from health and social perspective). So for adults it’s not so much the need for signalling that plays a role but a more rational decision weighing the benefit of getting high with different products against their ease of access and cost. From a public health perspective, it seems to me that the most effective way to decrease substance abuse is to decrease this demand to get high rather than targeting specific products. But why do humans have this demand for drugs? What does evolutionary biology say about that?

    Great blog by the way. You should post more often.


    1. Thanks for the kind words! I’m not sure I agree with the argument that keeping pot illegal provides a ready outlet for costly signaling that the grownups know is not very dangerous. For one thing, even though they make decisions in weird ways, teenagers are not altogether unsophisticated – they can sense inauthenticity pretty well. Also, let’s not forget about the other social consequences of the war on drugs, namely, the mass incarceration that has devastated urban communities. I think that the data is now clear that marijuana is simply not that dangerous when used responsibility. It’s on par with alcohol and possibly less dangerous even. I think the criminalization of it was a mistake all along. While harder drugs like cocaine and heroin are a different story.


  3. This is a really great point to make, but I wonder if trying to use costly signalling alone as THE explanation for this result after a drug policy change is a bit of a “just so story” explanation. It seems to me there are other reasons for trying / using drugs besides just signalling. Don’t they dull pain, or liven up experiences? The “rat town” experiment showed rats get addicted to drugs when their environments are impoverished. Maybe all the teens in America are just netflixing and chilling now. (I don’t really believe that. I’m just offering one confounding variable.)

    Liked by 1 person

    1. You raise a good point. My response would be that costly signaling is generally in play during the first few uses, and then other reasons come into play when it comes to persistent use. The best example is cigarette smoking. The first cigarette, even the first several, are NOT pleasant. There is really no “reward” except for the signaling. But once you get past the coughing, light-headedness, etc., you can get hooked. No one like cigarettes initially, but many get addicted. BUT… almost no one STARTS smoking over age 25, and very few even over age 21. It’s something that adolescents will try, but few adults will. Most adult smokers are those that got hooked while adolescents. That’s why the age restriction at least makes some sense in theory. If we can just get kids into adulthood without trying cigarettes, they won’t pick up the habit later. That’s the thinking.

      Liked by 2 people

    2. Ed: I like both your remarks and Nathan’s on this matter, and hold no brief against legalization — quite the opposite — but I have an additional concern. “Costly signaling” as an evolved behavior with a heritable basis clearly _could_ explain some or all of a behavior like taking risky drugs (whether the risk arises from the drug itself or from its illegality, as with marijuana), but that does not show that it _does_ explain it — i.e., that some biological, heritable factor is indeed causally involved, either in a tributary or dominant way. To know that that is so, to assert that this is a fact, don’t we need data showing that the behavior is indeed physically (not just culturally) heritable to at least some degree? And do such data exist for humans? If not, I submit that it is an overreach to say that this mechanism explains the behavior. It would be correct, and the difference is profound, not a quibble, to say that this mechanism _could_ explain the behavior.


      1. Yes, Larry, you’ve nailed my point with a bit more precision. The “just so story” criticism of explanations from evolutionary psychology is exactly about the difference between *can* and *does*.

        Liked by 1 person

  4. Despite scientific evidence… Thank you for speaking out about this. We also see this denial of science-based research in Scott Pruitt’s push back on regulating pesticides and the Department of Energy and the Pentagons desire to bring back nuclear arms.


    1. “Bring back nuclear arms” — er, had they gone somewhere? Despite some reductions, the US and Russia (these two primarily) have maintained extinction-level nuclear arsenals without intermission from the 1950s to the present. And this genocidal readiness is a bipartisan project: the Trump administration’s nuclear plan is actually Obama’s $1 trillion nuclear “modernization” program plus a few new trimmings. –> https://www.bloomberg.com/opinion/articles/2016-12-23/trump-s-nuclear-boast-is-obama-s-modernization-plan Trump’s withdrawal from the Intermediate-range Nuclear Forces treaty does amp up the madness, though.

      In any case, I don’t think that the wisdom or lunacy of having nuclear arsenal is a question that has been or can be settled by “scientific research.” Nuclear policy is a question of overt risks and consequences: of giant bombs going off by the thousands. One doesn’t need _science_ to know what happens when a pistol is fired through the back of one’s head . . . although science can supply some interesting details.


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