While masturbation may not lead to blindness or the growth of hair on one's palms, as people have been told for hundreds, likely thousands of years, excessive masturbation appears to be linked to the growth of an extra digit on the primary hand used in masturbation. Usually, this is the dominant hand.
A study conducted at Arkansas Southern University's Jocelyn Elders School of Medicine shows that excessive masturbation in men results in a 50% greater chance of growing an extra finger. Women who masturbate excessively were no more likely than a control group of typical masturbators to grow an extra finger.
Dr. Limerick Handy, who chaired the study, says that the growth of the extra finger is likely an adaptive biological strategy to compensate for extra use of the hand.