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The elusive nature of smiling, Millicent H. Abel smiles, lies, and emotion, Mark G. Frank the behavioural ecology view of smiling and other facial expressions, Alan J. Fridlund What do smiles mean? An analysis in terms of differential emotions theory, Jo Ann A. Abe et al smiling in infancy, Laura H. Bolzani et al smiling in children - displays and their meaning, Karen Caplovitz Barrett gender differences in smiling, Judith A. Hall who to whom and why - cultural differences and similarities in the function of smiles, Ursula Hess et al the therapeutic effects of smiling, Millicent H. Abel and Rebecca Hester conclusion - the not so elusive nature of smiling, Millicent H. Abel.
A Sat, study studied this question.