"These days we take for granted that we should smile when our picture is being taken." So write University of California Berkeley researchers, who note that photographers used to tell their subjects to say "prunes" instead of "cheese." That's because it was more fashionable to keep the mouth small in photos 100 years ago. But the so-called "smile intensity metric" has indeed gone up in the decades since, a fact they established after analyzing 37,921 high school yearbook photos dating back to 1905, creating average male and female images, and watching lip curvature change over time, reports Engadget. After computing changes in lip curvature, they found a "rapid increase in the popularity and intensity of smiles in portraiture from the 1900s to the 1950s."
At Smithsonian, Marissa Fessenden isn't particularly surprised by the findings, pointing out that early photography required subjects to hold a pose during what was a long exposure period, and holding a serious expression was easier than holding a smile. What is perhaps surprising is that the researchers found that women "significantly and consistently" smile more than their male counterparts, which gels with previous research that identified larger smiles among women in posed photos. The Sydney Morning Herald points out that, per the study, men registered a -0.5-degree lip curvature in 1905, meaning they were actually slightly frowning. The researchers next hope to use their visual historical dataset to study things like the actual "cycle-length of fashion fads." (One school in Utah caught flak for secretly editing girls' yearbook pics.)