Could test tube babies be revealing some of the hidden workings of evolution? It’s a definite possibility, judging from some recent reports about the balance of males and females.

For several decades, evolutionary biologists have been trying to figure out the forces that set this balance. It appears that they come down to a tug of war between competing interests. Imagine a species in which a freakish mutation makes the females gives birth to lots and lots of daughters. If you’re a male, suddenly your chances of reproducing look very good–certainly better than all those females. Now imagine that some of these lucky males acquire mutations that makes them father more sons than daughters. This son-favoring mutation would spread because