Women have always been mildly promiscuous throughout human evolutionary history. How do we know this? Unlike physical artifacts or skeletal remains, human social behavior like mating does not leave fossil records. So how do we know that women have not been sexually exclusive to their husbands throughout human evolutionary history? It turns out that clear evidence of women’s promiscuity has been left on the man’s body, his genitals, to be precise.

The first piece of evidence is the relative size of the testes. Across species, the more promiscuous the females are, the larger the size of the testes relative to the male’s body weight. This is because when a female copulates with multiple males within a short period of time -- in other words, when she is promiscuous -- sperm from different males must compete with each other to reach the egg to inseminate it. This process is known as “sperm competition.” One good way to outcompete others is to outnumber them...