As of January 2018, 29 U.S. states have legalized for medical use. Another 14 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 the 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 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 legalization, nationwide teen drug use is at a 20-year low.

Source: gomopix/Pixabay

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 backward.

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.

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 . 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.

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 when they are alone. This makes sense from an evolutionary perspective. Costly signaling only works when there is an audience of peers.

Source: Marc St. Gil/U.S. National Archives

Most drug deterrence initiatives repeat the refrain that drugs are harmful and taboo. But this is precisely what makes them so attractive to teens, 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.

The opposite is also true. When marijuana is legalized, and society acknowledges that the drug can be enjoyed safely, 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 who are desperate to prove themselves to their peers. When marijuana use is legal and safe, 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 . 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 of the Department of Justice, drug policy appears to be taking shape despite the scientific evidence, rather than because of it.