Consider an ingenious study by a team of four computer scientists at Brown and Berkeley. They took advantage of a neat digital-era development: many high schools have scanned their historical yearbooks and made them available online. Across the internet, the researchers found 949 scanned yearbooks from American high schools spanning the years 1905–2013. This included tens of thousands of senior portraits. Using computer software, they were able to create an “average” face out of the pictures from every decade. In other words, they could figure out the average location and configuration of people’s noses, eyes, lips, and hair. Here are the average faces from across the last century plus, broken down by gender:

Notice anything? Americans—and particularly women—started smiling. They went from nearly stone-faced at the start of the twentieth century to beaming by the end.

So why the change? Did Americans get happier?

Nope. Other scholars have helped answer this question. The reason is, at least to me, fascinating. When photographs were first invented, people thought of them like paintings. There was nothing else to compare them to. Thus, subjects in photos copied subjects in paintings. And since people sitting for portraits couldn’t hold a smile for the many hours the painting took, they adopted a serious look. Subjects in photos adopted the same look.

What finally got them to change? Business, profit, and marketing, of course. In the mid-twentieth century “century, Kodak, the film and camera company, was frustrated by the limited number of pictures people were taking and devised a strategy to get them to take more. Kodak’s advertising began associating photos with happiness. The goal was to get people in the habit of taking a picture whenever they wanted to show others what a good time they were having. All those smiling yearbook photos are a result of that successful campaign (as are most of the photos you see on Facebook and Instagram today).

But photos as data can tell us much more than when high school seniors began to say “cheese.” Surprisingly, images may be able to tell us how the economy is doing.”

Excerpt from ‘Everybody Lies: What the Internet Can Tell Us About Who We Really Are’ by Seth Stephens-Davidowitz,  2017, HarperCollins.