Several hypotheses have been proposed to explain the occurrence of continual sexual receptivity and concealed ovulation in human females. In view of the large number of benefits that would accrue to females if they could sense their own ovulation, these explanations appear insufficient to explain why ovulation is concealed from females as well as from males. The hypothesis presented here is that the phenomenon occurs because of a hominid female tendency to avoid conception in biologically nonadaptive ways. This tendency was countered by natural selection by making ovulation virtually undetectable to women. The sequence of evolutionary adaptations culminating in concealed ovulation is most likely as follows. First, olfactory, visual, and pronounced behavioral cues to ovulation were lost to conspecifics. Coincidentally females evolved continuous receptivity, frequently copulating at times other than when ovulating. Finally, females lost conscious cues to their own fertility. This last step was predicated up...