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The study addresses risk factors and cause of pediatric sensory over-responsivity (SOR) in a large sample of twins. At age 2 years, (a) the association of temperamental traits with concurrent SOR; (b) the association of prenatal complications with SOR; (c) the association of having a male cotwin with female SOR; and (d) the common and unique genetic causes of temperament and SOR symptoms are examined.The sample included 1026 twin pairs (mean age = 2 years 2 months) from a population-based longitudinal study. Auditory and tactile SOR symptom domains were partially independent and thus were examined separately.Temperamental negative affect and fear were moderately correlated with auditory and tactile SOR symptoms. Prenatal complications significantly predicted tactile symptoms after controlling for child characteristics. In addition, females with a male cotwin showed greater SOR at age 2 years than same-sex female dizygotic twins, suggesting a possible risk associated with in utero testosterone exposure. Both auditory and tactile SOR domains were heritable. Bivariate genetic analyses showed that each SOR domain had a similar genetic relationship with fear and negative affect.The findings suggest partially nonoverlapping causes and risk factors for tactile versus auditory SOR and indicate that prenatal factors warrant further investigation.