Evolution And Addiction


  • Randolph M. Nesse

    1. The University of Michigan Departments of Psychiatry and Psychology and Institute for Social Research 426 Thompson Street Ann Arbor MI 48104, USA
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Proximate explanations

An evolutionary approach to behavior is wonderful because is poses fundamentally new questions. It is also difficult, however, precisely because these questions are new and therefore hard to formulate clearly and even harder to answer. The papers in this special issue (Dudley 2002; Gerald & Higley 2002; Hill & Chow 2002; Lende & Smith 2002; Newlin 2002; Panksepp et al. 2002; Sullivan & Hagen 2002) tackle the problem of substance abuse from an evolutionary perspective. They range from deductions about risk-taking based on life history theory to adaptationist hypotheses about the possibility that drug use has been useful in our past, and therefore selected for. Those who are unfamiliar with such questions will appreciate a sketch of the larger framework in which evolutionary and proximate explanations are both needed to fully understand any biological trait.

Most substance abuse research has addressed the problem of why some people abuse drugs and others do not. The core challenge has been to explain individual differences. This focus arises from practical concerns—if we know the factors that predict who will abuse substances we should be able to change them and thus assist prevention and treatment. Much of the debate in the field has been between those who emphasize the importance of genetic differences and those who call attention to the roles of life experiences and psychological factors. Much of the progress has come from studies of the brain mechanisms—how they are changed by drugs and how they lead to drug craving and using. All of these questions and answers are fundamentally the same, however, in that they examine how things work. Together, they constitute what evolutionary biologists call a proximate explanation for substance abuse.

Why are we all vulnerable?

What they leave out, however, is the separate question of why all humans are vulnerable to substance abuse. This is a different kind of question, an evolutionary question about why we are all the same in this regard. Its answer is to be found in the forces of natural selection that shaped the brain (Nesse 1994). Substance abuse happens because natural selection shaped behavior regulation mechanisms that are based on chemical transmitters. Not only is it unsurprising that certain substances stimulate the system in ways that generate escalating drug taking, it seems nearly inevitable. While extremely simple, this principle is also important because it changes our perspective. Drug taking is what we should expect. The amazing thing to be explained is that substance abuse is so limited. Instead of asking what is wrong with people who are especially vulnerable to drug use, we should ask why some people do not use drugs, and especially why some people can use drugs and then stop.

This perspective also helps to explain why substance abuse is so devastating. If it took simply some time and money it would be more like gambling, video games or other unproductive diversions. Psychoactive substances, however, disrupt the very emotions that evolved to regulate our behavior (Nesse et al. 1997). They arouse reward mechanisms artificially, thus stimulating the circuits that are normally fired by an event that provides a huge gain in fitness; but they provide no fitness gain, they simply create an illusion. People who have few sources of pleasure in their daily lives are understandably more vulnerable to substance abuse. Treatments directed to the person’s whole life and place in a social network are essential because restoring normal sources of satisfaction may reduce the pull of drugs. Interestingly, an evolutionary approach supports a comprehensive and integrated search for the proximate causes of substance abuse, and it supports psychosocial treatments in conjunction with pharmacological interventions. Widespread adoption of an evolutionary view of addiction could be enormously useful by simply defusing useless controversies between different camps in the field, and by providing a biological foundation for integrated treatment models.

Why do plants carry drugs?

The second major question that emerges from an evolutionary perspective on substance abuse is why drugs of abuse exist ready-made in plants. They are there for the good reason that plants cannot run away from other organisms that want to eat them, so they have to rely on chemical defenses. Over hundreds of millions of years, plant biochemistry has been shaped to make chemicals that protect from herbivores. Some such chemicals, such as those that clog the mouthparts of insects, do not stop us for a minute. Others, such as tannins, impair digestion for both insects and mammals; but some chemicals, such as caffeine, cocaine and opiates are present in plants specifically because they interact with animal nervous systems. A mouse that tries to eat a coffee bean does not just suffer insomnia; it is likely to become very sick indeed. A leaf-miner that tries to eat a tobacco leaf does not become high, it dies. Nicotine, opiates and cocaine are potent toxins. Plants with such toxins have a selective advantage.

There is, however, another way that plants can obtain an advantage, and that is by offering something attractive. The nectar of flowers lures bees who spread pollen, and the tasty flesh of fruits means that they are eaten so the seeds are spread far from the plant, often reaching the ground in conjunction with rich nutrients. Are there any plants that contain drugs because they attract caretakers? Certainly tobacco, cocaine and opium poppies would not be nearly so widespread were it not for human desires for their chemicals. It seems likely to me that these chemicals evolved as toxins and the plants that make them simply happen to have had recent good luck in that they lead to human cultivation, but others have different ideas about this.

A related question is whether exposure to drugs has been a force of natural selection for humans. A mutated version of the gene that codes for alcohol dehydrogenase (ALDH2*2) has long been thought to offer a selective advantage by making its bearers sick when they drink alcohol. The relative frequency of this mutation in Asian populations, as high as 50%, may be explained by the centuries during which distillation has been available there. The other side of this idea is advocated by those who suggest in this issue that drug taking has benefited humans for long enough to have shaped special aspects of our psychology to reap those benefits and minimize the costs.

Whether or not selection has shaped some genes that prevent or confer benefits from drug use, it seems clear that we were never designed to cope with ready access to a wide variety of pure drugs. In the ancestral environment drugs were harder to obtain and harder to store and use. Now, every decade brings us new drugs, new methods of transporting them and new routes of administration. In this sense, drug abuse is a disease of civilization. This provides yet another source of wonder that drug use is not far more common than it is. Most young people now assume that normal people can resist addiction and only other weak people succumb. If the public perception was based instead on an evolutionary perspective that we are all designed in ways that make us vulnerable, perhaps this could foster a more realistic view of the risks.

Looking for an integrative framework

The questions addressed in this special issue are new and challenging. The perspective they offer is not an alternative, but complementary to other knowledge about substance abuse. I am not at all sure they will lead quickly to new ways to prevent or treat substance abuse, but they certainly do offer a useful foundation for understanding what substance abuse is, and why we all should be interested in the evolutionary reasons why we are vulnerable. Even now, however, this understanding may well provide a framework that can integrate different perspectives and provide a foundation for comprehensive programs of treatment and sensible public policies.