Why Do We Sneeze When We Look at the Sun?

A recent Scientific American on-line article discusses recent speculation on the photic sneeze reflex (PSR) - the reflex many people have to sneezing when staring at a bright light.

The article itself describes theories about the mechanics of this reflex, e.g., interfering nerve pathways leading to the brain, but there is no thought given to what evolutionary benefit, if any, there may be behind PSR. Several commentators observe that babies especially seem prone to this effect. I certainly noticed this myself: whenever I opened the shades in my children’s room in the morning they would almost invariably let out a sneeze - especially if they had a cold.

Here’s my theory: I assume that this reflex evolved as a way for young children to clear their nasal passages in the morning at the first light of day. In a time of no artificial lighting, the bright morning sun would be a good trigger for a young baby to clear mucus that had gathered throughout the night - like clearing the sand out of its eyes. That the PSR persists through adulthood is just a side effect of this; it certainly isn’t harmful and probably has some minor benefit.

Or course proving this is another matter entirely: is there any actual benefit to be had? I’ve always felt that this sort of evolutionary speculation is the last bastion of the arm-chair natural philosopher. If it can be shown that young children (those less able to clear their nasal passages themselves) actually have a higher incidence of this reflex, that would provide at least some clue that this is correct.