Mechanisms of predator avoidance in humans: Developmental, Neural, and Evolutionary contributions

Imagine you are hiking through a wood and you come across a long, thin, coiled object just behind the base of a tree. Most likely, you would be startled, your heart rate and blood pressure would increase, your muscles would tense, and you would either stay completely still or run away. These responses would arise rapidly, without any thought, in reaction to the danger signals that this object is, perhaps, a snake.

The kind of behavior displayed in the above situation is ubiquitous in the animal kingdom: All species play the dual roles of the hunter and the hunted, and the ability to respond quickly and appropriately to a potentially dangerous predator can make the difference between life and death. But how do animals know which species are potentially harmful? The theories of evolutionary psychology suggest that it is particularly adaptive for the basic mechanisms for predator avoidance to be present at birth: After all, learning about the potentially dangerous nature of a snake through a negative interaction may not allow an individual to pass this information along to the next generation. Moreover, along with finding food, predator avoidance was doubtless the most important problem to be overcome for our hominid ancestors, and indeed it is still a significant issue for a multitude of cultures in other parts of world where animals such as snakes take the lives of countless people each year. In support of this view, research with animals has suggested that many species do indeed have innate mechanisms to deal with predator avoidance. Toad tadpoles, for example, that have had no direct experience with predators, show an antipredator response to three species that predate upon them - backswimmers, waterbugs, and garter snakes - but not to two species that find them unpalatable (trout and newts).

Very little is known, however, about predator avoidance mechanisms in humans, and in particular, whether such mechanisms are present from birth or are learned through the course of development. This study proposes that infant humans, like other animals, are born with mechanisms that allow them to detect and identify particular stimuli as dangerous.  While it may seem counterintuitive to suggest that infants are born with such specific knowledge (e.g., snakes and spiders are dangerous, but frogs and rabbits are not), there is a great deal of evidence to support a role for innate mechanisms in the detection of predators by humans.  For many of us living in the Western hemisphere, predator avoidance seems a trivial issue; yet, the most common phobias among adults are of snakes and spiders. Why is it that so many individuals experience an intense fear reaction to snakes, creatures that are far more likely than not to be completely benign, whereas so few people have a similar reaction to a gun, which is much more likely to be dangerous?

The aim of this project is to examine infants’ and adults’ responses toward particular species of dangerous animals. As such, it is the first of its kind to investigate empirically the idea that infants are born with a mechanism that allows them to recognize potential predators. This view is a departure from accounts that hold that phobias, as well as less intense fears, result from conditioning whereby an object, entity, or place is accompanied by a negative experience, producing a powerful fear response to similar stimuli in the future. However, the idea of “fear conditioning” cannot account for people’s apprehension for things they have never experienced or have experienced but not in a negative way. The approach taken in these studies is that humans have evolved such that infants and children who responded – by crying, running, and so on - to dangerous predators were more likely to survive than those that did not, and the genes of these organisms would therefore be passed on to the next generation. While the proposal of innate mechanisms for predator detection is unique, there is ample evidence that humans are born with a number of similar psychological mechanisms that help infants to overcome various problems that are crucial for survival. For example, newborn infants reflexively suck and root (head turning to a tactile stimulus) which helps them to eat, and  neonates just one hour old look longer at a face-like image than at a scrambled version of that image, suggesting that they possess an innate predisposition to orient toward potential caregivers.

The hypothesis of the present investigation is that infants possess an innate psychological mechanism that allows them to recognize animals that were threats to our hominid ancestors – in particular, snakes and spiders. The experiments will investigate the capacity of neonates to recognize and respond to images of predatory animals.  They will also examine whether or not infants categorize predatory creatures together and differentiate them from other dangerous and non-dangerous animals.  The fear component of this process is hypothesized to be implicit at the outset – as indicated by an increased heart rate, for example – and becomes externalized once infants develop the emotional, motor, and cognitive skills to respond appropriately (by “freezing”, “fleeing”, or crying).  A second goal of this study is to examine the functional neuroanatomy involved in predator identification in phobic and non-phobic adults.  Specifically, these experiments will examine whether or not information about predators resides in focal regions of the neocortex of the brain, distinct from regions that identify other dangerous stimuli – guns for example – and nondangerous animals.  Another study will use functional brain imaging to determine whether or not basic perceptual mechanisms can detect predator stimuli and activate fear related structures in the brain without conscious awareness of the stimuli.

To date, these predictions have not been examined, partly because of the prevalent view concerning fear conditioning, partly because of ethical concerns about showing infants stimuli which we as adults know to be emotionally arousing, and partly because the appropriate methodologies for infant testing have not been available. However, using modern techniques that measure the extent to which infants orient to look at a stimulus, as well as techniques that allow accurate measurement of infants’ categorization, it is now possible to perform such studies. In a previous experiment using one such procedure,  10-month-old infants who had been shown five different pictures of snakes looked only briefly at  a new picture of a snake or a picture of a spider, but looked significantly longer at a rabbit or a frog. The implication of this finding is that infants were treating the snake and spider as equivalent and as different from the frog and rabbit. It is unlikely that these responses were based on the visual similarity of the animals involved – after all, a snake and a spider look very different – or that infants at this age had learned that some of these animals are dangerous and some are not.

This finding provides tentative evidence that the processes of evolution have given rise to a mechanism that helps in the detection of particular animals that have been dangerous to man in our evolutionary past. It also suggests that there may be other undiscovered mechanisms possessed by infants that help them to survive during the first few years of life, and it raises the possibility that early learning might be based, in part, on these innate mechanisms. Understanding that humans are born with predator avoidance mechanisms can also help to explain why children and adults develop phobias for some things rather than others: Agoraphobia, for example, might well relate to the increased danger from predators that our hominid ancestors experienced in open spaces. More generally, this work has strong implications for the so-called nature-nurture debate that focuses on the relative impact of genetically determined and learned bases for behavior. This debate has continued for hundreds of years, and this study proposes innovative methods to examine the relative contributions of these factors in a complex area of human behavior.  This information may not only be informative to our growing understanding of cognitive development, but may provide a basis for prevention and treatment of anxiety disorders throughout the lifespan.

For Further information
The following articles and books address the various issues raised by this project. All are accessible to a lay audience and should be available through non-specialist bookstores.

Infant perception, cognition, and face processing:

Kellman, P.J., & Arterberry, M. E. The cradle of knowledge: Development of perception in infancy. Cambridge, MA: MIT Press.

A clear and general introduction to the perceptual and cognitive abilities of infants.

Johnson, M.H., & Morton, J. (1991). Biology and cognitive development: The case of face recognition. Oxford: Blackwell.

A superb presentation of the author’s work on face processing that appeals both to a scientific and non-scientific audience. Face processing by other species is also discussed.

Fear and the brain:

LeDoux, J. (1994). Emotion, memory, and the brain. Scientific American, 270, 32-39.
LeDoux, J. (1996). The emotional brain. New York, NY: Touchstone.
Damasio, A. (1994). Descarte’s error: Emotion, reason, and the human brain. New York: Grosset/Putnam.

Excellent, easy to read introductions to the origins of emotion in the human brain. LeDoux, in particular, discusses how fear may have evolved to enable humans and other species to survive.

Evolutionary Psychology:
Barkow, J. H., Cosmides, L., & Tooby, J. (Eds.), (1992). The adapted mind: Evolutionary psychology and the generation of a culture. New York, NY: Oxford.

A collection of essays by leading evolutionary psychologists. It is a little advanced in places, but covers a wide range of areas of evolutionary study such as mating, play, co-operation, and language.