High school picture day is a tradition that dates back to the beginning of the 20th century. And while the basic formula of a front-facing pose against a neutral background hasn’t changed much in the past 100 years or so, facial expressions certainly have. For a better look at the similarities and differences between yearbook photos through the decades, researchers from the University of California, Berkeley recently compiled data on tens of thousands of images from the past 110 years [PDF].

The team's research marks one the first times machine-learning algorithms have been used to mine historic data from a collection of photographs. The high school yearbook photos were gathered from the digital databases of local libraries across the U.S. After downloading 150,000 pictures, the team eliminated all the photographs that weren’t front-facing portraits, leaving them with 37,000 samples from over 800 yearbooks from 26 states.

Next, they generated an “average” image for the men and women of each decade. By superimposing the groups of portraits together, they were able to then identify any trends exhibited in the composite images. One of the more interesting changes these pictures showed was the evolution of the subjects’ smiles.

When photography was still gaining popularity at the turn of the 20th century, it was normal for people to adopt a neutral expression for their portraits. According to the authors of the study, the earlier expressions might were partly a product of the beauty standards of the time: "Etiquette and beauty standards dictated that the mouth be kept small—resulting in an instruction to 'say prunes' (rather than cheese) when a photograph was being taken." (Not, it should be noted, due to long exposure times. By the late 19th century, innovations in photography had cut exposure time down to mere seconds.) 

As the 20th century wore on, the classic posed smile we adopt today began to emerge. An algorithm measuring the yearbook photos' shifting degrees of lip curvature supports this, and the trend can also be clearly observed in the composite images. Notably, this was around the same time that Kodak began releasing advertisements of happy people "smiling for the camera." The authors of the paper suggest this had a significant impact on how people chose to pose for their pictures.

The process also revealed data-backed proof of shifting trends in other areas, such as average hairstyles for each decade. While women of the '30s rocked finger waves, pin curls dominated the '40s and '50s, Afros took over in the '70s, and perms and bangs had their moment in the '80s and '90s. The data the team gathered on the progress of male fashion was slightly less interesting: men have been wearing standard suits in their high school yearbook pictures for over a century. 

Images Courtesy of the Cornell University Library.

[h/t: MIT Technology Review]