Infertility and The Mark of Gideon.
"Over the past 50 years, sperm counts have been declining in humans, in animals, even fish," [Serge Belisle, a professor of obstetrics and gynecology at the Centre hospitalier de l'Universite de Montreal] said. "It's a reality we have difficulty grasping because it's a new reality for us. Now, 50 per cent of the problem is male. (But) we know more about the biological prerequisites of conception from the female standpoint than the male."
The drop in male potency is believed to be related to environmental toxins and drugs in the drinking water that are responsible for the "feminization" seen in animals and fish. While environmental toxins adversely affect sperm production, lifestyle habits can have an effect, too.
"Tobacco is a major killer for sperm," Belisle noted.
Other factors include sexually transmitted diseases and delaying childbirth to later years. The quality of human eggs and sperm declines with age. In Canada, one couple in five between the ages of 22 and 40 will have difficulty conceiving. For those older than 40, the difficulty jumps to one in every two.
This is one reason why concerns about the overpopulation of the planet are not a cut-and-dried equation.
In my opinion the quantity of the Earths population is not going to be a real issue in this century - The quality of our lives will. Poverty, malnutrition, disease ... we have enough Horsemen of the Apocalypse waiting in the wings already.