I was always told it was a reaction to a lack of oxygen in the brain, If you were in a room that lacked oxygen then that could explain why other people all seem to yawn at the same time.
however, Im suspecting that the above may be old wives tales
as always wiki shows us the way...
no honest lol, not taking the pee, I just presume when i do it im bored but im sure thats not the right answer, unless im borng everyone else . seems every site has wierd and wonderfull names. anyway great to join you all
PJ
apparently it is because of a lack of oxygen to the brain - which happens when you are tired - and boring things give your brain a lack of stimulation that tires it, and then when someone sees you their brain is fooled into thinking that there isn't enough oxygen for you either, so it tries to do the same - and get us more air - how childish our bodies are :P hehe
xx
(I think I found that out on Brainiac)