Correlations between mating system and various aspects of genital anatomy suggest a strong influence of sexual selection on genital morphology. We test the generality of the influence by examining whether primate taxa in which there might be enhanced sexual selection (those with multi-male mating systems) possess, as expected, relatively more spinous penises than do taxa with other mating systems. As most prosimians, but few anthropoids (monkeys and apes), possess penile spines, and because the predominant mating systems of the two taxa differ, taxonomic constraints are taken into account. Sexual selection apparently does not act on penile spines in the same manner as on other aspects of genital anatomy: spinosity is not greatest in multi-male taxa of either prosimians or anthropoids. In some taxa, spines might stimulate reproductive readiness and synchrony in situations in which the sexes live apart and do not have other means of communicating reproductive state (dispersed social systems and `stolen' extra-pair copulations), but problems exist with the hypothesis, as they do with the idea that spines are involved with scent marking. It seems that either penile spines have several functions, or penile spinosity in primates, and other orders, remains to be explained.