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According to a study released by Evolution and Human Behavior researchers, we fake laughter often because it is important in communicating. The reason we laugh is to send a signal to other that we are listening and cooperating and want to be friends and are not threatening. That found that laughs at a faster speed are real laughs while slower speed laugh are more likely to be fake.
SOURCE:http://www.today.com/health/why-we-fake-laughter-how-tell-difference-2D79630152
