Abstract Cortical response patterns evoked by a light stimulus were studied for identical and fraternal twins and for age and sex-matched control subjects. The evoked potential (VER) responses were analyzed with a dual-channel on-line active band-pass filter system which separated the raw VER into the three common EEG frequency bands. The signals were next fed to the four input channels of a computer of average transients, CAT 1000. For each subject four X-Y plots representing the raw VER and the three major VER components were obtained. The visual evoked response determined from subdermal scalp electrodes is a stable and reliable physical measurement. Using only standard laboratory techniques the first 250 ms of the pattern after the stimulus is almost perfectly reliable for periods as long as 17 weeks. The hereditary nature of the visual evoked response is shown by the high degree of similarity of response patterns for identical twins and by the predictable decrease in similarity for fraternal twins and for unrelated control subjects. The concordance of the visual evoked response intra-class correlations for Mz and Dz twins with those of well established polygenetic, biometric and psychometric human characteristics is remarkable.