A new study from the journal Nature, has found that the older a man is when his child is conceived, the more likely that child is to develop autism or schizophrenia.
The research was gathered from 78 Icelandic families whose offspring was diagnosed with either of the two diseases.
It showed that the father's age at the time of conception accounts for the vast majority of new mutations found in the baby.
The average child of a 20-year-old man has 25 random mutations that could be traced back to paternal genetic material, where as the average child of a 40 year old has 65.
Sperm are susceptible to more of these genetic mutations than eggs because they are constantly being produced and experience more cell divisions. Sperm from older men carry more errors.
"We've known for some time that spontaneous mutations probably were significantly more common in children with autism, what we didn't know is how much more common" Doctor Thomas Insel, with the National Institute of Mental Health, said.
The research counters past assumptions that the mother's age was considered the key factor in determining the odds of a child having developmental problems.
Most people have many of these "de novo" mutations, which occur spontaneously at or near conception, and most of them are harmless.
But the more a child has, the more likely he or she is by chance to have one of these rare, disabling diseases.
Researchers have stressed that the results shouldn't keep older couples from starting families as a father's age still only accounts for about 5 percent of the risk of autism.