Well ... mostly because we haven't found yet such difference (that I know of) and sperm has been thoroughly studied. IVF in humans can be very telling if numbe

Well ... mostly because we haven't found yet such difference (that I know of) and sperm has been thoroughly studied. IVF in humans can be very telling if number crunched properly - the eggs are extracted and the mother body is out of the equation for a moment. Now if the sex selection/discrimination is population based - it is simpler and easier to make it in the males. If it is per embryo - than you have no choice but make it in the females no matter how complex.