Our response to the things that scare us, from threatening men on dark streets to hairy spiders in the bath, is programmed to become active at different times in our lives, suggest two studies on the genetics of fear.
Scientists already know that fears and phobias are shaped in part by genes. Identical twins, for example, are more likely to develop phobias for the same objects, such as snakes or rats, than non-identical twins. But less is known about when the genes involved act and what effect they have.
In the case of spiders, that effect may be hard-wired from birth.
David Rakison of Carnegie Mellon University in Pittsburgh, Pennsylvania, showed five-month-old babies simple representations of spiders, made up from block-like shapes, as well as more jumbled images made from the same shapes. The babies looked at the schematic spiders for an average of 24 seconds, but spent around 8 seconds less on the more jumbled images.
This suggests babies are born with a "mental template" for spider shapes, and potentially for other things that may harm them, say Rakison and his colleague Jaime Derringer of the University of Minnesota, Minneapolis, as it is unlikely that the babies developed a specific interest in spiders in the few months they had been alive.
For safe objects, that template appears to be missing. When Rakison repeated the experiment using a representation of a flower and a jumbled equivalent made from the same shapes, the babies looked at all the images for around the same length of time.
Once identified, spiders produce fear and disgust in many people. But that reaction, and the genes that shape it, change during our youth, according to a separate study by Kenneth Kendler of Virginia Commonwealth University in Richmond and colleagues.
They tracked a group of around 1250 pairs of twins, both identical and non-identical, through childhood and adolescence and used questionnaires to ask about the things that scared them.
Since twins grow up in similar environments, genes and environment play comparable roles in determining their fears. So if identical twins are more similar than non-identical twins, then that similarity can be attributed to their identical genes rather than the environment.
University of life
Kendler's team found that the genetic effect fluctuated as the children grew up. The genetic contribution to fears relating to blood and injuries peaked between ages 13 and 17, for instance.
The results do not say why different genes act at different times, but Kendler notes that a plausible explanation comes from the fact that our ancestors faced different kinds of threats at different points in their lives.
Getting lost in the dark might have been the most dangerous thing for a four-year-old, for instance. A young adult need not worry so much about getting lost, but would have had to fear strangers, since he or she would have been at risk of being attacked by members of another tribe.
Kendler's study confirms previous work showing that fears and phobias have a substantial heritable component, says Elliot Nelson, a psychiatrist at Washington University in St Louis, Missouri.
The results also show that clinicians may be able to prevent fears becoming ingrained by intervening at critical points during a child's development.
"It is extremely important for clinicians, particularly those working with children and adolescents, to remember that a patient's worldview is very dynamic during these important developmental periods," says Nelson.