A classical theory held that deuteranomaly was caused by the replacement of the normal M cone pigment by an anomalous M pigment, which was spectrally red-shifted. Two assumptions associated with that theory were: (I) that there was a single, fired L pigment, shared by deuteranomals and normals; (2) and that everyone had two genes for cone pigments on the X-chromosome. Those two assumptions are now known to be invalid and with regard to red-green color vision, an alternate theory, introduced by Alpern et al. in the 1970s, has enjoyed renewed attention. It holds that photopigment spectra are inherently variable and form "clusters." Those with normal color vision are supposed to have drawn one pigment front each cluster, but deuteranomaly arises when what would normally be the L and M pigments are both drawn from the L cluster: Both the Alpern theory and the classical one continue to play important roles in current thinking. However we present evidence here from ERG flicker photometry that observed similarities and differences between the distributions of spectral sensitivity functions of deuteranopes and deuteranomalous trichromats are intermediate between what might be predicted by Alpern's theory and what would be predicted by the classical theory. (C) 2000 John Wiley & Sons, Inc. Col Res Appl, 26, S106-S111, 2001.