Tick-tock, tick-tock. Is that your biological clock? And if it is, are you female? Most people think biological clocks are strictly female phenomena, with infertility and the risk of certain diseases going up with a mother's age. But the biological clock is an equal opportunity timepiece--that's right, men have one too!  There's a common belief that men can sire children until they die, but that women need to be relatively young to conceive healthy children. But the father's age affects the child's health too. Psychiatrist Dolores Malaspina and her team found that the risk of schizophrenia triples for babies born to fathers over 50, compared with fathers under 25. Other diseases appearing more in offspring of older fathers are achondroplasia, or dwarfism, and some prostate cancers.

Despite the publicity given to women's biological clocks, medical geneticists recently have found that sperm are more likely than eggs to have genetic mutations. Here's why. By age 40, a man's sperm cells have divided over 600 times. That's over 600 chances for mistakes in copying genes. A woman's eggs, on the other hand, don't go through that process of division, so there are fewer chances for mistakes in gene replication. When there is a genetic problem in an egg, though, it's often with a whole chromosome, instead of a single gene. Down syndrome, for example, occurs when there's an extra copy of chromosome 21. Most babies with older fathers, as with older mothers, are healthy. Still, biological clocks are real, and not just for women. His biological clock is ticking too!