During the study, scientists found that baby girls only 11 months old rapidly start to associate pictures of spiders with fear. However, baby boys remain blithely indifferent to this connection.
In an initial training phase, David Rakison, a developmental psychologist at Carnegie Mellon University in Pittsburgh, Pennsylvania, showed 10 baby girls and boys a picture of a spider together with a fearful face.
In the following test phase, he let them watch the image of a spider paired with a happy face, and the image of a flower paired with a fearful face.
Despite the spider's happy companion, the girls looked significantly longer at it than at the flower. The researchers took this to mean that the girls expected spiders to be linked with fear. The boys looked for an equal time at both images.
With a different group of babies, Rakison first showed a spider with a happy face, and a flower with a fearful face. Now the girls too looked at both images for the same length of time - implying that they did not have an inborn fear of spiders.
The results suggest that girls are more inclined than boys to learn to fear dangerous animals.
On the other hand, modern phobias such as fear of flying or injections show no sex difference, Rakison said.
He attributes the difference to behavioral differences between men and women among our hunter-gatherer ancestors. An aversion to spiders may help women avoid dangerous animals, but in men evolution seems to have favoured more risk-taking behaviour for successful hunting.
It makes evolutionary sense to acquire spider fear at a certain age, rather than to be born with it, Rakison said.
"There is little reason for an infant to fear an object unless it can respond to it, for example by crawling away," New Scientist quoted him as saying.
The study has been published in the journal Evolution and Human Behavior.