So this is the 'theory' Male sperm has a Y chromosome, which is actually just a truncated X chromosome. Y chromosomes are therefore ever so slightly lighter than X chromosomes. Y sperms travel slightly faster as a result, Y and X sperms can be separated using centrifuging (one way of sex selection in other countries) due to this difference in weight. This is all proven and true. it goes on... X chromosomes are slightly slower and very slightly more sluggish, and as a result, they last longer (say 4-5 days?) Whereas Y sperm are nippier and don't live as long. This is more debatable as true imo. SO If you use ovulation sticks, you basically see that you're fertile and have sex as soon as it says positive. So the sperm most likely to get to the egg in time is the male sperm. so if you want a girl, you should try say 3-4 days prior to ovulation. That way only the female sperms will be alive when you ovulate. Opposite is true, if you want a boy, wait until the day you ovulate.