Most people believe that yawning is most powerfully triggered by seeing someone else yawn  This belief about yawning is widespread not only today, but also has been commonplace in many parts of the world in the past, if we are to believe historians of popular culture
Thus, seeing someone else yawn must be the most irresistible cause of yawning
It makes a sweeping generalization about yawning based on evidence drawn from a limited number of atypical cases.