We estimated the proportion of people that have defective stereo vision and are unable to utilize stereo disparity information to perceive depth. Previous estimates have ranged anywhere from as high as 30% to as low as 6%. Our goal was to understand the basis for the wide range in these estimates. To do this, we administered two psychophysical tests to a sample of 100 young adults. Visual stimuli consisted of dynamic random-dot stereograms presented using a fast-decay, time-sequential display device. The stimuli covered a range of disparities between 0 and .38 degrees (both crossed and uncrossed). A forced-choice methodology was used to estimate whether subjects could perceive depth based on horizontal disparity. It was found that display duration was a key variable determining the number of viewers that were classified as stereo-anomalous. The relatively high incidence of stereo- anomalous viewers in previous research was explained by the short display durations (80 ms) used in those studies. With longer durations of about 1 sec, we found that only about 5% of viewers had defective stereo vision.