According to a study in 2006, female servers who draw a smiley face on the back of a customer’s bill receive 20 percent more in tips. The same is not true for male servers.
Sonoran Conservative
Conservative of the Southwest and the musings of the day. – Murum Aries Attigit
Sonoran Conservative 
Correlation does not equal causation.
Maybe friendlier servers are more inclined to put smiley faces on the back of receipts.
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