Girls but not boys may be primed for arachnophobia, ophidiophobia: fear of crawly, slithery things could begin before first birthday.

Gut-wrenching fears of snakes and spiders may start early for many women. Before their first birthdays, girls but not boys adeptly learn to link the sight of these creatures to the frightened reactions of others, a new study suggests.

Neither infant girls nor boys link happy faces with snakes and spiders, reports study author David Rakison of Carnegie Mellon University in Pittsburgh in an upcoming Evolution & Human Behavior. Youngsters of both sexes also don't tend to associate images of flowers and mushrooms with either fearful or happy faces, he finds.

In Rakison's tests, 11-month-old babies first looked at pairs of images - a happy or fearful cartoon face was paired with a snake, spider, flower or mushroom. After the first brief display, Rakison timed how long each child gazed at new pairs of images. Youngsters who learned to associate two images, say a fearful face with a snake, would gaze longer at a violation of what they expected to see, the researcher reasoned.

Only girls associated the snake or spider that they originally saw with a fearful reaction and then acted on that knowledge, looking longer at the unexpected appearance of a happy face with a new snake or spider, Rakison proposes. No other pair of images elicited longer gazes from girls or boys.

If confirmed in further studies, these findings support the idea that people have evolved a brain mechanism that primes them for learning to pair fearful expressions with threats that would have repeatedly confronted prehistoric populations, Rakison says. In his view, bites from poisonous snakes and spiders presented a special danger to prehistoric women, whose children would have suffered or died without their mothers.

A Swedish survey of adults and children found that 5.5 percent report snake phobias and 3.5 percent report spider phobias. These phobias affect roughly four times more women than men.

"The basis for women's greater incidence of fear and phobias for snakes and spiders may be an evolved fear mechanism that operates during infancy and is especially sensitive in females," says Rakison.

Rakison theorizes that risk-taking personalities offered greater survival value for men in prehistoric times, when they had to hunt, defend their families and occasionally fight other groups.

A simpler form of learning may explain the findings, comments Vanessa LoBue of the University of Virginia in Charlottesville. Infants of both sexes may be equally primed to fear snakes and spiders. But if it-month-old girls are generally better at recognizing facial expressions than their male peers, that would give infant girls an advantage at pairing fearful faces with snake and spider images.

In an unpublished review of her earlier studies, LoBue concludes that 5-yearold girls recognize threatening and non-threatening expressions more quickly than boys. But it's not yet known if that difference holds for 11-month-olds. Further research needs to establish whether the sex difference reported by Rakison vanishes as boys become more experienced at decoding faces, LoBue says.