Original Article
Does women's greater fear of snakes and spiders originate in infancy?

https://doi.org/10.1016/j.evolhumbehav.2009.06.002Get rights and content

Abstract

Previous studies with adult humans and nonhuman animals revealed more rapid fear learning for spiders and snakes than for mushrooms and flowers. The current experiments tested whether 11-month-olds show a similar effect in learning associative pairings between facial emotions and fear-relevant and fear-irrelevant stimuli. Consistent with the greater incidence of snake and spider phobias in women, results show that female but not male infants learn rapidly to associate negative facial emotions with fear-relevant stimuli. No difference was found between the sexes for fear-irrelevant stimuli. The results are discussed in relation to fear learning, phobias, and a specialized evolved fear mechanism in humans.

Introduction

The evolved function of fear is to organize responses when confronted with a particular kind of adaptive problem, namely, danger. Over evolutionary time, specific dangers were recurrent, and there would have been intense selection pressure for the emergence of psychological mechanisms that facilitate fear learning for them (Öhman & Mineka, 2001). Two recurrent dangers to humans from other species throughout human and primate evolution were spiders and snakes (Öhman & Mineka, 2003). Fear of these nonhuman animals is common in adults and children, and they elicit phobias in approximately 5.5% (snakes) and 3.5% (spiders) of the population (Fredrikson, Annas, Rischer, & Wik, 1996). Moreover, there is a consistent sex difference in the incidence of snake and spider phobias; women are four times more likely than men to have fears and phobias for these, but not other stimuli (e.g., injections, heights, and flying) (Fredrikson et al., 1996, Marks, 1969, Rose et al., 1981). Is part of this greater incidence of snake and spider fear in women the result of a specialized evolved fear mechanism? This question was examined in the two experiments reported here.

It is particularly important for fear learning of specific threats to be facilitated by an evolved psychological mechanism because it is not adaptive to learn about the potentially dangerous nature of, for example, snakes and spiders by being bitten and killed. According to Öhman et al., 2001, Öhman and Mineka, 2001, human and nonhuman primates possess an evolved fear mechanism for fear-relevant stimuli such as snakes and spiders that is selectively responsive to and is triggered by such stimuli. This fear mechanism predisposes children and adults to attend to snakes and spiders and prepares them to rapidly to learn to associate the appropriate emotional response—namely, fear—with such stimuli.

There is now considerable evidence from adults, children, infants, and nonhuman primates to support this view. For example, human adults show superior conditioning for images of snakes and spiders with a mild shock than for fear-irrelevant stimuli such as flowers, mushrooms, or electric outlets (Öhman & Mineka, 2001). Human adults and children also more quickly detect snakes against a background of flowers or mushrooms than flowers or mushrooms against a background of snakes (Lobue & DeLoache, 2008; Öhman et al., 2001). There is also evidence that 7- to 18-month-old infants (both boys and girls) associate snakes with fear because they look longer at movies of snakes paired with a frightened human voice than movies of snakes paired with a happy human voice (DeLoache & Lobue, 2009). Furthermore, infants at 5 months of age look longer at a schematic image of a spider than a partly or completely scrambled image of a spider but do not do so for schematic and scrambled images of a flower (Rakison & Derringer, 2008). Finally, there is also evidence that rhesus monkeys reared in the laboratory associate snakes with a fearful response from another monkey more quickly than they associate flowers with a fearful response (Cook & Mineka, 1990).

According to Error Management Theory (EMT) (e.g., Haselton & Buss, 2000), many judgment and decision-making adaptations are designed by natural selection to be biased to err in the direction of lower survival or reproductive cost. This view predicts that humans' fear mechanism may be designed to be particularly sensitive to, or prepared for, pairings of negative emotions and specific recurrent threats because the fitness cost of not learning such pairings would be high [see also Nesse's (2001) “Smoke detector principle” that defenses are often expressed too readily or too intensely]. It is plausible that this may be the reason why so many individuals develop an irrational aversion, or phobia, of a recurrent threat, which is consistent with the fact that phobias tend to be related to evolutionarily relevant stimuli (e.g., snakes, spiders, heights, and people).

Why, then, should women be more likely than men to develop phobias for snakes and spiders? There are a number of plausible explanations for this sex difference. One possibility is that social transmission of fears and phobias is more common or promoted among women than men (Fredrikson, Annas, & Wik, 1997). Alternatively, women's fear mechanism may be more sensitive to snakes and spiders than males' fear mechanism because women were more exposed to them over evolutionary time (e.g., during child care, while foraging and gathering food). It is also feasible, as predicted by EMT, that a fear of snakes and spider was particularly important in women because it protects both their child and themselves. In other words, the fitness costs of being bitten by a snake or spider would have been greater for women than for men because infants and young children, historically, rarely survived a mother's death (Buss, 2008). Finally, because of the higher reproductive variance for men, evolution would have selected against males with overly powerful fears because it could have inhibited risk taking involved in, for example, large game hunting.

The current experiments were designed to establish whether, indeed, the basis for adult females' greater incidence of fear and phobias for snakes and spiders is rooted in an evolutionary mechanism that is present in infancy. According to EMT, the nature of this evolved mechanism would lead female infants to show an advantage over male infants in learning associations between snakes and spiders and a negative facial emotion but would not do so for fear-irrelevant stimuli. Alternatively, work by DeLoache and Lobue (2009) in which no sex differences were found suggests that both boys and girls should show an advantage for learning associations between recurrent threats and a negative facial emotion relative to associations between non-threats and a negative facial expression. It is also plausible that an evolved fear mechanism is agnostic about the emotion to be paired with a recurrent threat, which suggests that boys and girls should show an advantage for learning associations between recurrent threats and any facial emotion relative to learning associations between non-threats and a facial expression. Note that although any of these findings would provide evidence for the presence of an evolved fear mechanism in infants, they would not necessarily rule out any of the evolutionary explanations presented above for why women are more likely than men to develop fears and phobias for recurrent threats. Finally, if fear learning is unrelated to a specialized evolved mechanism and is underpinned by more general learning mechanisms, infants should not show any difference in learning associations between facial emotions and threats or non-threats.

Section snippets

Experiment 1

In this experiment, 11-month-old infants were tested in the visual habituation paradigm with an adaptation of the Switch design. Infants were habituated to a single color photo of a spider or a snake paired with either a happy or a fearful schematic face. In the test phase, infants were presented with a novel spider or snake paired with a different face (e.g., happy if habituated to a fearful face) as well as a mushroom or flower paired with the same novel face. As outlined above, if infants

Participants

Participants were 20 healthy full-term infants with a mean age of 11 months 9 days (range: 10 months, 13 days to 11 months, 22 days). There were an equal number of boys and girls. An additional 8 infants were tested but not included in the final analysis because of failure to habituate (n=5), experimenter error (n=1), or looking more than 2 S.D. beyond the condition mean (n=2).

Materials and design

During the pretest, infants were shown two stimuli (snake or spider and mushroom or flower) one at a time to determine

Discussion

These data show that girls, but not boys, who were habituated to a fearful face and a recurrent threat looked significantly longer when a novel snake or spider was paired with a different facial emotion relative to when a mushroom or flower was paired with the same facial emotion. This suggests that female infants associated the snake or spider seen during habituation with the fearful facial emotion and generalized it to the novel snake or spider in the test trials. The same effect was not

Experiment 2

An alternative explanation for girls' looking pattern in Experiment 1 is that they are highly attuned to learn the pairing between a negative emotion and any stimulus. If this were the case, the results of the first experiment would be unrelated to learning about the pairing of a fearful emotion with snakes or spiders per se. To test this explanation, I designed Experiment 2 to be identical with the first experiment except that infants were habituated to a single color photo of a

General discussion

The two experiments reported here show that female 11-month-olds—but not males of the same age—learn the relation between a negative facial expression and fear-relevant stimuli such as snakes and spiders. Importantly, the same effect was not found for paired associate learning between facial emotions and fear-irrelevant stimuli such as mushrooms and flowers. As such, the current data support the hypothesis that women may be more predisposed than men to learn the appropriate emotion for nonhuman

Acknowledgments

The author would like to thank Gabriel Smith, Jessica Jankowitsch, Katie Andreasson, and the rest of the staff of the Infant Cognition Laboratory at Carnegie Mellon University for their help with data collection and participant recruitment. He would also like to thank two anonymous reviewers and Martie Haselton for excellent suggestions and critiques during the review process.

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This work was supported by a grant from the National Institute of Child Health and Human Development (R03HD049511-01).

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