Face perception has often been investigated with human faces differing in categories such as race or gender. Here, we investigate the perceptual border across species. We applied a method based on support vector machines to generate images of hybrid monkey ^ human faces (`morphs') with different levels of human contribution. In the `explicit' experiment, we asked subjects to rate morphs at different morph levels as `humans' or `monkeys'. We found that subjects rated the morphs as humans when they had a human contribution of at least 56%3%. In the `implicit' experiment, we asked whether subjects could distinguish between successively presented morphs differing by 10% morph level from a morph centre. By varying the morph centre value from 10% to 90%, we were able to measure subject's sensitivity to detect species differences along the human ^monkey continuum. We found that the sensitivity of subjects to detect species differences was highest when morphs had a human contribution of 65%3%. In summary, the human ^ monkey boundary does not lie at the midpoint of the human ^monkey continuum, but tends to be shifted towards the human side. Our results reveal an asymmetry in the perception of human ^monkey morphed faces, which may be species-specific and/or due to expertise.