"ON their first exposure to the laughter, nearly half of the students reported that they responded with laughter themselves. (More than 90% reported smiling on first exposure.) However, the effectiveness of the stimulus declined with repetition. By the 10th exposure, about 75% of the students rated the laugh stimulus as "obnoxious", a reminder of the sometimes derisive nature of laughter, especially when repetitive and invariable."
(Beyond a joke: the truth about why we laugh; Guardian)