If snakes strike terror in your toddler’s heart, he might still grow to be brave. A tendency toward fearfulness does have genetic underpinnings, but those shift several times as children become adults, a study has found.
The worries of adolescents differ from those of young children — fear of the dark gives way to squeamishness about blood in a well-documented developmental progression. Now, psychiatrist Kenneth Kendler of the Medical College of Virginia in Richmond and his colleagues have found that the genetic factors that leave a person prone to fear also shift during development.
To tease apart the effect of genes and upbringing, the researchers tracked 2,490 Swedish twins as they aged from 8 to 20 years old, asking them to answer questions sent by mail. The twins were quizzed on whether they were afraid of 13 potentially terrifying phenomena, including lightning, dentists, spiders and heights.
At every age a child was more likely to be fearful if their identical twin was too. Fraternal twins also shared a tendency towards fearfulness but the link was less strong, indicating a genetic component to fearfulness.
However, despite this evidence for a genetic effect, children weren’t consistently prone to fear as they grew up. Evidence for multiple fear factors comes from the comparison between ages - some twins were similarly fearful at age 8, but not at older ages.
Similarly, young adult responders who were easily frightened were no more likely to have had a fearful identical twin during early adolescence, the team reports in the Archives of General Psychiatry 1. “You might have fairly substantial changes in levels of fearfulness over time because different genetic effects are coming online at different ages,” Kendler says.
The genes that contribute to fearfulness at different ages remain unknown, as evidence for the shift lies entirely in the strength of the links between fear levels in identical twins across time.
Work to identify specific genes for such complex traits is in its infancy, says psychiatrist Murray Stein, who studies the biological underpinnings of anxiety at the University of California, San Diego. “We are starting to see findings of specific genes being associated with particular kinds of temperaments,” Stein says, but he notes that variation in any of these genes explains only a very small percentage of variability in human behaviour.
Stein thinks that the study highlights the importance of recognizing that the factors that lead to excessive fears, or phobias, may change over time. “It could be that we’re going to need very different interventions at different stages of people’s growth and development,” he says.