"Aaaaaaaahhhh!!!!!" The mere sight of a snake or spider strikes terror in the hearts of millions of people.
A new study suggests that such fear has been shaped by evolution, stretching back to a time when early mammals had to survive and breed in an environment dominated by reptiles, some of which were deadly.
"Their brains certainly had to be effective in identifying reptiles in the world around them," said Arne Öhman, a psychologist at the Karolinska Institute and Hospital in Stockholm, Sweden.
Öhman, who was afraid of snakes and spiders as a child, is co-author of a study on how the human fear of snakes and spiders evolved. A report on the research appears in the September issue of the Journal of Experimental Psychology: General.
The research shows that mammals developed the perceptive ability to focus on things seen as threatening, such as snakes and spiders, and to respond emotionally with a feeling of fear.
To reach their conclusions, Öhman and his colleagues conducted experiments in which their subjects were asked to identify pictures depicting sources of fear (snakes and spiders) from among an array of images that did not induce feelings of fear (mushrooms and flowers), and vice versa.
A statistically significant number of the subjects found the pictures of the snakes and spiders more quickly than they did those of mushrooms or flowers.
Joseph LeDoux, a professor of neural science and psychology at New York University in New York City, said the results of the research by Öhman and his colleagues were generally accepted within the scientific community. "Certainly there are certain stimuli that are pre-wired in the brain because they have been perennially dangerous to our ancestors," he said.
The test subjects, many of them psychology students at the Karolinska Institute, were seated in a soundproof chamber in front of a screen on which the researchers presented a slideshow of pictures.
The subjects held switches in their hands as they watched the slideshow and clicked the switches when they identified the snakes and spiders or mushrooms and flowers. The researchers recorded the amount of time it took for the subjects to react to the various pictures. As a control, some of the picture collections had no snakes and spiders or, vice versa, no mushrooms and flowers. Among the results, the researchers found that where in the picture collection the images of snakes and spiders appeared was not relevant to the amount of time it took to identify them, whereas the location of the mushrooms and flowers in the photo array was related to the speed with which they were identified.
This time difference, according to the researchers, suggests that the feared objects "popped out" from the display and were detected more automatically.
In a related experiment, the researchers found that people who had indicated on a questionnaire that they were afraid of snakes or spiders identified the fear-inducing images even faster than they identified the objects that did not evoke fear. This quicker response by people with a phobia about snakes and spiders is an emotional reaction that enables them to better avoid the objects they fear, the researchers said.
"Evolution has equipped mammals with a readiness to easily associate fear to recurrent threats in their evolution," said Öhman. "Thus, given that fear is activated when a snake is around, they condition fear to the snake much easier than to other stimuli that are around."
Evolution of Perception
LeDoux said that while the research is sound, it's difficult to make any conclusions about the evolution of perception with utmost confidence because there is no fossil record.
Richard McNally, an associate professor of psychology at Harvard University in Cambridge, Massachusetts, agreed. "The biggest challenge that we face in considering these theories of evolution is we cannot recover the evolutionary history—there is no fossil record," he said. "People seem to have different thresholds for saying what is a plausible account of evolution."
McNally questions, for example, whether mammals really have evolutionary cause to be afraid of spiders. Only 0.1 percent of the 35,000 different kinds of spiders in the world are poisonous, he noted.
Öman acknowledged that more research is needed to bear out the findings of the new study, but he contends that a fear of snakes, at least, seems to be shaped by evolutionary influences.
"Snakes have provided a recurrent threat throughout mammalian evolution," he said, adding that "individuals who have been good at identifying and recruiting defense responses to snakes have left more offspring than individuals with less efficient defense systems."