Psychologists found a strong correlation between high IQ and healthy sperm, as reported by the U.K. Telegraph:

The research, by the evolutionary psychologist Professor Geoffrey Miller of the University of New Mexico, centred around a study of 400 Vietnam War veterans who were put through extensive mental tests and were also asked to provide sperm samples. According to the test results, it was found that men who scored high on a battery of intelligence tests boasted high counts of healthy sperm. Whereas, low scorers tended to have fewer and more sickly sperm.

Miller points out that intelligence is sexually attractive to both sexes, and that intelligence co-evolved with sperm quality to advertise good genes.