A longitudinal twin study of fears from middle childhood to early adulthood: evidence for a developmentally dynamic genome.

CONTEXT While the nature of common fears changes over development, we do not know whether genetic effects on fear-proneness are developmentally stable or developmentally dynamic. OBJECTIVE To determine the temporal pattern of genetic and environmental effects on the level of intensity of common fears. DESIGN Prospective, 4-wave longitudinal twin study. Structural modeling was performed with Mx. SETTING General community. PARTICIPANTS Two thousand four hundred ninety twins and their parents from the Swedish Twin Study of Child and Adolescent Development. MAIN OUTCOME MEASURE The level of parent- and/or self-reported fears obtained at ages 8 to 9, 13 to 14, 16 to 17, and 19 to 20 years. RESULTS Thirteen questionnaire items formed 3 distinct fear factors: situational, animal, and blood/injury. For all 3 fears, the best-fit model revealed developmentally dynamic effects and, in particular, evidence for both genetic attenuation and innovation. That is, genetic factors influencing fear intensity at age 8 to 9 years decline substantially in importance over time. Furthermore, new sets of genetic risk factors impacting fear intensity "come on line" in early adolescence, late adolescence, and early adulthood. As the twins aged, the influence of the shared environment declined and unique environment increased. No sex effects were found for situational fears while for animal and blood/injury fears, genetic factors in males and females were correlated but not identical. Shared environmental factors were both more important and more stable for animal fears than for situational or blood/injury fears. CONCLUSIONS Genetic effects on fear are developmentally dynamic from middle childhood to young adulthood. As children age, familial-environmental influences on fears decline in importance.