A Longitudinal Twin Study of Fears From Middle Childhood to Early Adulthood
Evidence for a Developmentally Dynamic Genome
- Mack, A. MD
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 (Fig 2).
FIGURE 2.
—The proportion of total variance in fears accounted for by genetic factors through development. The y-axis represents the total phenotypic variance so the sum of all the factors equals the total heritability. Purple represents the first genetic factor starting at age 8 to 9 years. Blue represents the second genetic factor starting at age 13 to 14 years. Yellow represents the third genetic factor starting at age 16 to 17 years and green represents the fourth genetic factor acting only at age 19 to 20 years. A, Results for situational fears, the exact parameter estimates of which are seen in Table 5. B, Results for animal fears, the exact parameter estimates of which are seen in Table 6. C, Results for blood/injury fears, the exact parameter estimates of which are seen in Table 7. (Reprinted from Kendler KS, Gardner CO, Annas P, et al. A longitudinal twin study of fears from middle childhood to early adulthood: evidence for a developmentally dynamic genome. Arch Gen Psychiatry. 2008;65:421-429. Copyright 2008 American Medical Association.)