Breeders of animals have long been familiar with the fact that among nonsterile individuals there exist all grades and degrees of fertility, from the lowest to the highest. The recent application of this concept to human beings has clarified several aspects of the problem of sterility.
Unless the fertility of a mating rises above a certain threshold, that mating will be barren. Thus a husband and wife, both of low fertility, may remain childless, even though either partner could produce offspring by a highly fertile mate. Low fertility, which is not in itself sterility, is now known to be a definite and fairly common cause of sterile marriage.
Relative fertility is primarily a feature of the sex cells themselves, rather than the result of conditions in the genital passages. A considerable variety of factors can so affect the gonads that only relatively infertile spermatozoa or ova are produced.