
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.

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.
Correlation does not equal causation.
Maybe friendlier servers are more inclined to put smiley faces on the back of receipts.
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