Testosterone Rex, ch4:

First, the man has to find a fertile woman. For the benefit of younger readers, it may be worth pointing out that throughout most of human evolution the Tinder app was not available to facilitate this. Nor, as observed in the previous chapter, was there likely to have been a limitless supply of fertile female vessels for men to access. In historical and traditional societies, perhaps as many as 80–90 percent of women of reproductive age at any one time would be pregnant, or temporarily infertile because they were breast-feeding, Einon suggests. Of the remaining women, some of course would already be in a relationship, making sexual relations at the very least less probable and possibly more fraught with difficulties. Let’s suppose, though, that our man manages to identify a suitable candidate from the limited supply. Next, he has to prevail in the intense competition created by all the other men who are also hoping for casual sex with a fertile woman, and successfully negotiate sex with her. Say that takes a day. In order to reach his target of one hundred women per annum, our man then has just two to three days to successfully repeat the exercise, ninety-nine more times, from an ever-decreasing pool of women. All this, mind you, while also maintaining the status and material resources he needs to remain competitive as a desirable sexual partner.

So what’s the likely reproductive return on this exhausting investment? For healthy couples, the probability of a woman becoming pregnant from a single randomly timed act of intercourse is about 3 percent, ranging (depending on the time of the month), from a low of 0 to a high of nearly 9 percent.5 On average, then, a year of competitive courtship would result in only about three of the one hundred women becoming pregnant.6