While the aging sperm machine may be one theory, McCulloh says there is an equally strong possibility that it may not be the aging process at all that's at fault, but rather a man's lifestyle that affects his fertility - and that what a man does over his lifetime that matters most.
"It is very difficult to separate the effects of natural aging from environmental effects like those from smoking, excessive alcohol, social drug use, and radiation exposure. There are a whole battery of environmental assaults that may accumulate over time, causing at least some of the reproductive issues we now attempt to link solely to age," says McCulloh.
Silverman agrees, suggesting that much the way lifestyle practices affect the health of other systems in the body, such as the heart, so too might they affect male reproduction. "The longer a man lives, the more exposure he has -- which might make a difference," says Silverman.
At least one study indicated that oxidative damage -- one type of environmental effect-- can increase chromosomal damage in sperm.
In animal research published in 2005 in the journal Fertility and Sterility, researchers found that not only was the DNA of sperm susceptible to oxidative damage, but the older the male, the more susceptible the sperm was to developing breaks in the DNA.
It is this kind of genetic upset that experts say could be behind some of the birth defects and other problems that until recently were thought to be exclusively related to the mother.
The American Society of Reproductive Medicine now recommends that sperm donors be men that are "ideally less than 40 years of age to minimize the potential hazards of aging."