What is the cost of sex? Put simply, it is the cost of males.

Consider two populations: one is asexual, and one is fully sexual. Both have a stable sex ratio of 1:1. For simplicity, suppose that each female produces two offspring. In the asexual case, the population will double every generation, since each member of the population produces two offspring. In the sexual case, the population will remain constant, since half of all offspring are males, and only contribute to the next generation by fertilizing the females.

The clone of offspring from an asexual female multiplies at twice the rate of the progeny descended from a sexual female, and a sexual female has only 50% of the fitness of an asexual female. John Maynard Smith (pictured opposite) has devoted a great deal of time to the issue and described it as the twofold cost of sex.