There is a theory that male sperm swim faster than female sperm, but die more quickly. Whether the egg will be fertilized by a male or female sperm, then, depends on how far down the fallopian tube it is when sex occurs. If it's close to the uterus, then the male sperm get there first. If it's father up the tube, then the male sperm die before they get there, and the female sperm do the fertilization. All one has to do in order to determine the sex of the child, then, is know when ovulation occurs, and time sexual intercourse accordingly.
This has worked in at least three cases that I know of. A friend of mine introduced me to it. He and his wife used it to produce a girl. My wife and I used it to produce a son. He knows of one other couple that used it successfully. In all three cases, the desire was to even out the male/female ratio in the family.
What this suggests regarding the male/female ratio is that, given an equal number of male and female sperm cells, and a random distribution for sexual activity, the ratio will be roughly even. It is, in fact, 105:100, rather than 100:100, but close enough.
