Women are likely to be different. Having sex with a stranger not only encumbered a Pleistocene woman with a possible pregnancy before she had won the man’s commitment to help rear the child, but it also exposed her to probable revenge from her husband if she had one and to possible spinsterhood if she did not. These enormous risks were offset by no great reward. Her chances of conceiving were just as great if she remained faithful to one partner, and her chances of losing the child without a husband’s help were greater. Therefore, women who accepted casual sex left fewer rather than more
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