As I said yesterday, the book Evolutionary Biology of Human Female Sexuality takes a standard view on human promiscuity:
Humans have been selected to pair bond, express mutual mate choice, and biparentally care for offspring. In these respects we appear to have diverged substantially from our closest phylogenetic relatives.  … Though in many species of nonhuman primates mating with multiple males to reduce their maltreatment of offspring has importantly selected for extended female sexuality, women’s extended sexuality probably does not significantly reflect this function. 
But in the range from near-perfect monogamy to chimp/bonobo style promiscuity, where do they think humans lie? Most of the book talks about how the signs of various effects suggest our ancestors had some “extra-pair mating,” but their only concrete indication of magnitudes is to describe the Hadza:
Hadza men spend much more time hunting large game than returns to their families warrant, which implies that male Hadza foragers do not … maximize gains through parental investment. So long as opportunities for mating with women other than currently primary partners are available to men … men should not be expected to allocate foraging time … [for] the fitness benefits of parental effort alone. …
Overall married Hadza women produce as many calories as do married Hadza men. … Women whose youngest children are 3 years of age or younger harvest about one-third fewer calories. And women with infants 1 year of age or younger harvest only about half as much. … Their husbands make up for the shortfall. 
So a Hadza man hunts big game to look sexy, even though that retrieves less food. Except that when a women he has sex with has a kid he thinks is his, he’ll gather more but less-sexy food, to give this woman ~1/2 of her food for one year, ~1/4 for the next two years, and declining amounts thereafter.
Now, yes, this may be more pair-bonding than in chimps or bonobos. But it is also far less than the farmer ideal of life-long monogamy! Many men today reluctant to marry for life would be ok with this level of commitment. And of course we don’t know if Hadza levels of pair bonding were typical of our distant ancestors.
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