A few of our fears, however, are hardwired into our brains before we are even born.
Girls know as young as eleven months old associate snakes and spiders with fear (boys don't have that instinctual association at that age).
[T]hese findings support the idea that people have evolved a brain mechanism that primes them for learning to pair fear expressions with threats that would have repeatedly confronted prehistoric populations. . . . In [the study author's] view, bites from poisonous snakes and spiders presented a special danger to prehistoric women, whose children would have died or incurred great hardship without their mothers.
Surveys of adults and children find that 5.5 percent report snake phobias and 3.5 percent report spider phobias. These particular phobias affect roughly four times more women than men.
The study (a small one with just twenty subject) examined how infants response to pictures of different facial expressions in the presence of spiders and snakes.
"Males also develop fears and phobias about snakes and spiders" but the fears aren't manifest as early on and the phobias are less common.
Others reviewing the experimental results, offer an alternative explanation.
Another research notes that the gender differences seen in the study could also make sense if an inborn fear of spiders and snake was identical, "if 11- month-old girls generally recognize facial expressions better than their male peers." The infant study measured fear associations with spiders and snakes using pictures of faces with different expressions on them. This researcher found in her own work that 5-year-old girls recognize both threatening and non-threatening facial expressions more quickly than boys.
One of her recent studies also notes that some of the fear response may be to "snakes’ slithering motion" in particular. The study showed that "7- to 18-month-olds of both sexes look longer at movies of snakes while listening to frightened, versus happy, voices. But the same infants did not look longer at still images of snakes paired with a frightened voice, compared with snake images accompanied by a happy voice." In her view:
From an evolutionary perspective, it makes the most sense for both boys and girls to learn quickly to fear threatening stimuli such as snakes and spiders, but gender differences in fear learning warrant further investigation.
Either way, the most notable point to me is that the study confirms the widely known fact that we are born with and have access at a very early age, to a small set of instinctual fears that was evolutionarily relevant to our ancestors. While most of the knowledge we acquire during life is a matter of nurture, a small part of our memory is ancestral.
In every day life these days, some of those instinctual fears don't have a lot of evolutionary value. Dangerous snakes and spiders aren't a major threat to modern man. I suspect that some of the things we instinctually fear don't even exist anymore, in the wake of massive megafauna extinctions coincident with the rise of human and human ancestor populations on Earth. Instead, their main impacts these days seem to be phobias and the perennial popularity of certain horror movie cliches.
But, it is worth recalling that we have our instinct driven "animal" side, one which we share with all mammals. I've noted before that a disproportionate share of mental health issues seem to be associated with the part of our brain that all mammals share.
Even more intriguingly, much of what we commonly think of as especially "human" about us is also associated with this part of our brain, rather than the part that is actually unique to humans. For example, many people think of a capacity to love family members and experience fear, which are traits shared by most mammal species, as things that make us unique human. But, few people would consider species unique abilities like playing chess and translating never before seen text into language, to be quintessentially human. These days, that makes a certain amount of sense.
While in the "Age of Reason" the main question we faced in asking what it means to be human is the distinction between humans and animals, in the modern era, we ask the question mostly to distinguish between humans and computers.
The story of our struggle to understand what it means to be human these days is told more in classics like Arthur C. Clarke's 2001: A Space Odyssey, the 1983 movie War Games and Issac Asimov's I, Robot than it is by stories like Robert Louis Stevenson's Strange Case of Dr. Jekyll and Mr. Hyde that were relevant to an earlier era.