Friday, January 11, 2008

Spiders, Snakes, and Evolved Fears

Did you ever take a moment to think about some of the common phobias or fears among people and why they are so common? Scientists have wondered about this for some time, and a great deal of research has focused on two widely held fears, those of spiders and snakes. Why do so many of us fear spiders and snakes when 1) a disproportionate number of them are not dangerous to us, and 2) most of us have never had a dangerous encounter with one?

A knee-jerk answer to those questions might be that we have a culturally-induced fear of snakes and spiders, one that is reinforced by myths, legends, and the rare true story of poison-induced death. But research like the type conducted by David Rakison of Carnegie-Mellon University and Jaime Derringer of the University of Minnesota seems to contradict the idea that fears of snakes and spiders are (at least predominantly) culturally developed. They showed pictures of spiders to five-month old infants and measured the amount of time they spent fixated on the them vs. pictures of other objects. The infants spent an average of 7-8 seconds longer looking at the spider pictures, an indication that the spiders were more interesting to the newborns, at a time when they seemingly could not have learned the significance of a spider through cultural or environmental processes. Similar research has been conducted in the past with spiders and snakes, with similar results, although not with humans of such a young age.

So what does this mean? Do we have inborn fears of snakes and spiders? Well, perhaps we are not born with a fear of spiders, but maybe with a predisposition toward viewing snakes and spiders in a different way than we would a cat or a flower. The evolutionary viewpoint is that a predisposition toward certain fears that would increase the chances of survival would be passed on. Sure, most snakes and spiders we encounter today aren't harmful, but in the environment where our evolutionary ancestors dwelled (Africa) there were more harmful varieties, as well as little medical ability to manage a poisoning. Thus, an encounter with a snake or spider was something to be feared, and those that had a healthy fear would be more likely to survive, reproduce, and propagate their genetic information. Rakison and Derringer suggest that a genotype that increases the chances of survival by only 5% would be widespread throughout a population in 20-30 generations.

Behavioral predispositions can be selected for evolutionarily because they are grounded in the physiology of the brain. In the case of this discussion, that physiology probably involves the amygdala. The amygdala is an almond-shaped (amygdala means almond in Greek) group of neurons in the temporal lobe, and part of the limbic system. The limbic system is heavily active in emotional reactions, and the amygdala has been implicated to a large extent in reactions of fear. Thus, an evolved fear module has been proposed that centers around the amygdala. This fear module is thought to consist of a network of brain regions that has evolved to recognize particular threats, like spiders and snakes, with little cognitive processing. This enables protective behavior without having to sit around and think about whether the spider is a black widow, the snake is a copperhead, etc. (by the time you decide, you may already have been bitten) This fear module may predispose babies to recognize spiders and snakes, and might be responsible for our unreasonable fears of them. If correct, this theory could also provide explanations for why we are afraid of heights, the dark, water, and even public speaking.

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