  The second assumption is that there is greater variance in male than female reproductive success. While this is the case for many species, (for example, Bateman's fruit flies), it is an empirical matter whether or not this applies to other species. In some species, variance in female reproductive success is higher than has been assumed and in some species, variance in male reproductive success is lower than is often assumed. Hrdy (1981/1999, 1986) points out that there has been a lack of attention to ways that a female can end investment in a particular reproductive attempt, for example, female birds abandoning nests or spontaneous abortion among some species of mammals. Attention also needs to be paid to the effects of a female's physiological condition and social status on her reproductive output. Hrdy demonstrates that female primates have an impressive array of active strategies that they employ to control their own reproduction. For example, competition and systems of social alliances among female primates can lead to unexpected variance in female reproductive success. Hubbard (1990) allows that theoretically there could be greater variance in reproductive success among men than among women, but claims that generally most societies have the same number of men and women producing children and do not operate using a few “super-studs.” Hubbard points out that it remains to be demonstrated that weaker men, however one would measure this characteristic, have fewer children than powerful men and that women tend to have similar numbers of children.