Editing Concealed ovulation (section)

===Social-bonding hypothesis===

Schröder<ref name="Schroder 1993" /> presents the idea of a "gradual diminution of mid-cycle estrus and concomitant continuous sexual receptivity in human women" because it facilitated orderly social relationships throughout the menstrual cycle by eliminating the periodic intensification of male–male aggressiveness in competition for mates.<ref name="Schroder 1993" /> The extended estrous period of the bonobo (reproductive-age females are in heat for 75% of their menstrual cycle) has been said to have a similar effect to the lack of a "heat" in women.  While concealed human ovulation may have evolved in this fashion, extending estrus until it was no longer a distinct period, as paralleled in the bonobo, this theory of why concealed ovulation evolved has frequently been rejected. Schröder outlines the two objections to this hypothesis: (1) natural selection would need to work at a level above the individual, which is difficult to prove; and (2) selection, because it acts on the individuals with the most reproductive success, would thus favor greater reproductive success over social integration at the expense of reproductive success.

However, since 1993 when that was written, group selection models have seen a resurgence.<ref name="Koeslag, 1997">{{cite journal | last1 = Koeslag | first1 = J. H. | year = 1997 | title = Sex, the prisoner's dilemma game, and the evolutionary inevitability of cooperation | journal = J. Theor. Biol. | volume = 189 | issue = 1| pages = 53–61 | doi=10.1006/jtbi.1997.0496 | pmid=9398503| bibcode = 1997JThBi.189...53K }}</ref><ref name= "Koeslag, 2003">{{cite journal | last1 = Koeslag | first1 = J. H. | year = 2003 | title = Evolution of cooperation: cooperation defeats defection in the cornfield model | journal = J. Theor. Biol. | volume = 224 | issue = 3 | pages = 399–410 | doi = 10.1016/s0022-5193(03)00188-7 | pmid = 12941597 | bibcode = 2003JThBi.224..399K }}</ref><ref name="Wilson, D. S. 2008">{{cite journal | last1 = Wilson | first1 = D. S. | last2 = Wilson | first2 = E. O. | year = 2008 | title = Evolution 'for the good of the group'. [Article] | journal = American Scientist | volume = 96 | issue = 5| pages = 380–389 | doi=10.1511/2008.74.1}}</ref> (See [[group selection]], [[reciprocal altruism]], and [[kin selection]].)