Psychol Rep 2005 Oct 01;972:651-65. doi: 10.2466/pr0.97.2.651-665.

Smiling in photographs: childhood similarities between sexes become differences constant in adulthood.

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Contemporary published photographs of 1639 children, 200 older teenagers, and 304 adults in North America were analyzed for the smiling pattern (full, partial, or none) exhibited by individuals by sex. For each of the three age groups and for both sexes, most individuals posed with a full smile. No statistically significant differences in smiling pattern were present between the sexes for photographs of children at preschool and grade-school age. By teenage years, and even more for the adults, there were statistically significant differences between the sexes with regard to a smiling facial expression. In those cases, more female than male subjects smiled fully, whereas more male than female subjects did not smile. Examination of photographs of well-known persons longitudinally through adulthood showed that individuals tended to be consistent in smiling pattern. There was no significant sex difference for this relative constancy of facial expression in posed photographs.

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