Stephen ID, Law Smith MJ, Stirrat MR, Perrett DI (December 2009). "Facial Skin Coloration Affects Perceived Health of Human Faces". Stephen ID, Coetzee V, Law Smith M, Perrett DI (2009). "Skin blood perfusion and oxygenation color have an effect on perceived human health". Penton-Voak IS, Perrett DI (January 2000). "Female preference for male faces variations cyclically: Further proof". Jones BC, Little AC, Burt DM, Perrett DI (2004).