Sneezing may seem like a simple process, but it is actually set off by a remarkably complex chain reaction. A sneeze begins in the brain stem, where signals are dispatched through the nervous system that tell the eyes, mouth and throat to shut tight. Then in quick succession the abdominal and chest muscles contract while the throat suddenly relaxes. This one-two combo forces air, saliva and mucous out of the nose and mouth in an explosive eruption designed to expel contaminants [source: Hatfield].
So, what causes sneezing? Most often, it is allergies, followed closely by the common cold. Other primary causes include the involuntary nervous system (sneezing when exposed to bright light fits into this category), sneezing related to seizures, and psychological or emotional sneezing [source: Brody].
Fear, for example, causes the nasal membranes to shrink and this can make you sneeze. Other emotions like frustration, resentment, sadness or anguish can cause the nasal membranes to swell, which also can trigger a sneeze. The same is true of excitement, joy and sexual arousal.
It could be that if we are tuned in to others' emotions -- say the excitement of friends on the way to a concert or the grief of people close to us -- the very act of being empathetic and bonded with the group could result in similar emotion. And this emotion could lead to sneezing [source: Brody].
As scientists look for definitive answers as to whether and why social sneezing occurs, the animal kingdom may contain a few clues. Although the reasons for the response are still murky, baboons, chimpanzees and dogs have all been observed catching yawns from other members of their groups. No word yet on whether the same is true of a sneeze [source: Dell'Amore].
Even though humans and animals have been known to catch a yawn from each other, the phenomenon affects some people differently based on how aware they are of facial cues. For example, research has shown that yawns aren't contagious to children younger than 5 or to children who have autism. The reason is that they may not be as adept at noticing the facial cues of a yawn that would trigger a sympathetic reaction. Perhaps similar, yet uncharted, differences occur with adults, too. This would explain why some people are so apt to yawn (or perhaps sneeze) sympathetically and some aren't [source: Geggel, Preidt, Bakalar].
Of course, if you're dining with friends and the waiter's overzealous application of cracked pepper sends microscopic stimuli soaring toward everyone's nostrils, a chain-reaction sneeze fit may occur. But that's more about shared external stimuli than sympathetic sneezing.