There has been an unexplained drop in dizygotic twinning rates during the past 15--20 years in all the developed countries for which data are available. In this note an explanation is offered for this decline. 2) There has been a decline in birth rates in almost all developed countries over much of the same period. 3) There is good evidence that in recent years there has been a decline in sperm counts--at any rate in the United States. 4) It is suggested here that this decline in sperm counts has been responsible for a decline in the probability that coitus in the fertile period will result in fertilization of the ovum. It is hypothesized that this decline is responsible for the decline in dizygotic twinning rates and--partially--for the decline in birth rates.
