This study examined the contributions of genetic and environmental factors to the observed correlation between intelligence test scores and speed of information processing, based on data for same-sex adult twin pairs (age, 15-57). Verbal and performance IQ scores from the Multidimensional Abilities Battery, as well as 11 reaction-time measures derived from a battery of information-processing tasks, were available for 50 monozygotic and 32 dizygotic pairs of twins. Multivariate biometrical analyses were used to estimate genetic and environmental parameters underlying observed variances and covariances among intelligence test scores and a general speed of information-processing factor (based on a linear composite of the 11 reaction-time scores). A common-factor model with loadings on general speed of processing, verbal IQ, and performance IQ fit the data well. The common factor was influenced primarily by additive genetic effects, such that the observed relationships among the speed and IQ measures are mediated entirely by hereditary factors. There was additional specific genetic variance for Verbal IQ and specific shared-twin environmental variance for Performance IQ. However, twin similarity for general speed of processing was explained entirely by genetic factors related to intelligence. The results emphasize the importance of common, heritable, biological mechanisms underlying the speed-IQ association.