The culprit: The photic sneeze reflex, which usually causes more than one sneeze and occurs in approximately 10 to 25 percent of the population. This genetic trait is thought to occur due to an accidental crossing of third nerve signals in your brain at the fifth cranial nerve nucleus (remember that next time you're playing Trivial Pursuit). If you have a parent who is a sun sneezer, you have a fifty percent chance of being a sun sneezer as well.