Why Has Natural Selection Left Us So Vulnerable to Anxiety and Mood Disorders?

Abstract

What can evolutionary biology offer to our understanding of anxiety and depression? According to 2 articles in this issue, a lot.1,2 Both include details and debates that could easily obscure their shared crucial main point - the capacities for anxiety and mood were shaped by natural selection because they have been useful. Like sweating, pain, and cough, emotions are only useful in certain situations, so natural selection shaped them in tight conjunction with regulation mechanisms that express them when they are likely to be useful.3 High body temperature arouses sweating, tissue damage arouses pain, and foreign material in the respiratory tract arouses cough. People who lack these response capacities are likely to die young. So are people who express them too readily, too intensely, or too long. Regulation mechanisms have been finely tuned by millions of years of selection. However, spending a day in the clinic suggests that the designer of the systems that regulate anxiety and mood must have been having a very bad day. Any engineer responsible for such apparently slipshod design would certainly be subject to legal action! So, what is the problem? Is natural selection too weak to do better? Evolutionary medicine suggests that this is only 1 of 6 reasons why selection has left us vulnerable to diseases.4,5 Two others, emphasized in these articles,1,2 are that some conditions that seem like diseases are actually defences, and that every trait is subject to trade-offs; making it better in one respect will make it worse in others. Dr Melissa Bateson, Dr Ben Brilot, and Dr Daniel Nettle1 provide a sophisticated analysis of the trade-offs involved in regulating anxiety. They begin by explaining that evolutionary and mechanistic explanations are equal partners in any complete biological explanation. This foundation from basic behavioural biology should be familiar to readers; if not, the first chapter of any animal behaviour textbook will explain. The authors proceed to argue that anxiety can be useful. This is not controversial, and is already the foundation for much work on anxiety disorders,6-8 although research documenting the benefits of normal anxiety is overwhelmed by studies showing the costs of anxiety disorders. They next apply signal detection theory to calculate the optimal signal threshold for expressing an anxiety response. The mathematical foundation they provide is essential, but a simple example illustrates the smoke detector principle. You are a hunter-gatherer at a watering hole. You hear an animal behind a small hill. The noise could have been made by a lion, or by a monkey. Should you flee? Noises made by lions are generally louder, so it depends on how loud the noise is, the relative prevalence of monkeys and lions in the area, and the cost of fleeing versus the cost of not fleeing if a lion is really there. If the cost of fleeing - that is, a panic attack - is about 200 calories, and the average cost of not fleeing in the presence of a lion is about 200 000 calories, then you should flee if the sound is loud enough to make the probability of a lion's presence greater than 1/1000. This means that 999 times out of 1000 your panic response will be a false alarm, but nonetheless normal. When I first did this calculation, I could hardly believe it. Ever since, I have found it valuable in the clinic. I had previously explained to patients that their panic symptoms were not caused by heart problems but by a disease, panic disorder; most listened politely, and then went for more cardiac tests. Now I tell patients that a panic attack is a normal response that is useful in the face of life-threatening danger, and panic disorder results from false alarms in that system, some of which are to be expected, just as they are in smoke detectors. I also explain that experiencing panic attacks makes the world seem more dangerous, lowering the anxiety threshold, thus causing a vicious circle. …