In an attempt to clarify the genetic and environmental causes of sleep terrors in childhood, reasearchers in Canada followed 390 pairs of monozygotic and dizygotic twins by assessing the frequency of sleep terrors at 18 and 30 months of age using a questionnaire administered to the biological mothers. The prevalence of sleep terrors was 36.9% at 18 months and 19.7% at 30 months. Boys and girls were affected equally. The polychoric correlations were 0.63 monozygotic and 0.36 dizygotic at 18 months and 0.68 and 0.24 at 30 months. Sleep terrors were best explained by a genetic and non-shared environmental, 2-component model. At 18 months, genetic factors accounted for 43.7% and non-shared environmental factors for 56.3% of the phenotypic variance; at 30 months, these proportions were 41.5% and 58.5%, respectively. 
COMMNT. The prevalence of sleep terrors is high in infants (37% at 18 months) and decreases by one half to approximately 20% by 30 months of age. Genetic factors play an important role in the etiology of this early childhood parasomnia, accounting for >40% of the phenotypic variance for both 18- and 30-month-old twins. The role of non-shared environmental factors is also significant, >55% of the variance at both 18 and 30 months. Night terrors have a combination genetic-environmental etiology, but to date, no specific genes have been identified.