Dawkins mentions this in Unweaving the Rainbow (which is about debunking paranormal bollocks).
(Though I can't remember if he came up with this or if he's quoting someone else.) He says you could make a nice little earner by advertising your patent formula that promises to make your baby a boy for £500, and advertise a money-back guarantee plus £250 on top of that if your baby is born a girl. Since boys turn up on 50% of occasions anyway, you'll make a profit.
( Bob Todd whee, Thu 3 Apr 2008, 15:42, Reply)