I hope this also work for Human

Apparently canaries can choose the sex of their babies according to the song of the male.
The male canaries will hone a love song, and how "attractive" the song will determine the size of the eggs lay by the female canaries. larger egg = male birds. (Link)

That was easy! I mean if Human babies can be as simple as that, the earth will be fill with Einstein and Mozart and Super Heroes.. well, maybe not super, just heroes.