From Nature:
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.
More here.
