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Dr Baker proposed that men with bigger testicles would be most successful in the race to fertilise most eggs, and that the testis size of individual men could predict their success in sperm competition. Remarkably, Dr Baker had managed to persuade 14 of his male colleagues to measure the size of their left testicle, using callipers. He then asked 20 female colleagues to look at the men, and rank them according to whom they would most like to have an adulterous relationship with. As he predicted, there was a correlation between the big-balled males and the apparent likelihood that the men would engage in extra-pair copulation, if given the opportunity.

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