Using quantitative genetic research designs, we decomposed phenotypic variance in P300 parameters into genetic and environmental components. The twin method was used to carry out this decomposition. Event related potentials (ERPs) were measured during a visual oddball paradigm in a sample of 213 adolescent twin pairs. The presence of male and female same-sex and opposite-sex twins in the sample enabled us to study sex differences in the contributions of genetic and environmental effects to P300 parameters. For targets and nontargets, half of the variance in the P300 amplitude is attributable to factors shared by the family members. However, it remains unclear whether this resemblance is attributable to shared environmental or genetic influences. The same factors (genetic or shared environmental) were found to contribute to the individual differences in males and females. The contributions do, however, differ across gender. Multivariate genetic analyses investigated the covariance among various brain areas to determine whether the covariance between two or more leads is attributable to the same genetic and/or the same environmental factors. The covariance of the P300 amplitude measured at different locations was attributable both to unshared environmental and to shared factors. Again it was not possible to show that the shared factors where either genetic or shared environmental.