Sperm Counts!

NYT: "The children of older fathers scored lower than the offspring of younger fathers on I.Q. tests and a range of other cognitive measures at 8 months old, 4 years old and 7 years old, according to a study released Monday."
Other studies have found that children of older dads are statistically more likely to suffer from autism and schizophrenia.

Also interesting on the dad front: a guy's genes may favor producing more girls than boys (or vice versa).