Asked by: Caroline Hare, Kings Lynn
In metabolic terms, the cost of reproduction for women is very high, so it is evolutionarily better to abandon conception for a particular month than to press on when conditions are not favourable.
For example, if a woman is already pregnant, it’s obviously not to her advantage to compromise this pregnancy by implanting a second embryo. So as soon as a woman conceives, her hormone levels change to prevent more eggs being released and the female contraceptive pill can mimic these changes to fool the body.
For men, the cost of reproduction is much lower and natural selection wouldn’t favour men who became infertile just because they had already recently impregnated one woman. So men don’t already have a natural ‘off switch’ that a simple hormone pill can exploit. Instead male contraceptive pills have to interfere with sperm production or viability or inhibit ejaculation in a more artificial way. This causes a lot more problems with undesirable side effects.