Throughout the late 1960s and 1970s, the English artist David Hockney painted a series of "double portraits," two subjects depicted side by side -- of the fashion designer Ossie Clark with the textile designer Celia Birtwell; of the author Christopher Isherwood with the artist Don Bachardy; and one of his own mother and father, among others.

For nearly all of them, critics especially applauded Hockney's ability to convincingly paint faces, capturing both emotion and a sense of mysterious intimacy.

The one exception to these sympathetic, evocative depictions, however, is a painting titled "American Collectors (Fred and Marcia Weisman)," in which the Weismans' faces are slightly blurred and blunted to appear similar to the statues behind them.

It's not an obviously negative portrayal, exactly, but to most viewers, it seems clear that Hockney perceived his subjects as flat and dull, their faces signifying their well-bred background and high social class but also their utter lack of original artistic discernment.

The ability to convincingly and subtly depict social class through one's face alone is a rare skill -- not everyone can be David Hockney -- but the artist was also tapping into something more universal: Research suggests that the face alone might provide clues to someone's social class, especially for those who know how to look.

Generally speaking, we pick up hints on a person's socioeconomic status by looking for more obvious markers, things that are buyable or learnable -- clothes, watches, comportment, manner of speech. (Just think of the line in The Great Gatsby when Jay Gatsby turns to Nick to proclaim that Daisy's "voice is full of money.") There are subtler, academically proven cues , too, like the fact that someone from a higher class tends to be more disengaged and aloof when dealing with someone from a lower one.

But a study recently published in the Journal of Personality and Social Psychology posits that we really don't need any of these clues to know someone's social class: The face alone can be our guide.

"We know that people use facial information for a lot of other things," says lead study author Nicholas O. Rule, a psychology professor at the University of Toronto. We look at the face to make judgments on "someone's race, someone's sex, someone's age, and even more subtle things like sexual orientation, political affiliation, or religion. It seemed very logical to us, actually, that people would be able to get social class information from people by looking at their faces."

For their experiment, Rule and co-author R. Thora Bjornsdottir, a social psychology doctoral student at the University of Toronto, recruited 81 undergraduates to look at photographs of 80 Caucasian men and 80 Caucasian women between the ages of 18 and 35. The photos were sourced from dating websites from major U.S. cities, and the researchers removed all identifying signs from the faces -- cropping them, standardizing them in height, and putting them into grayscale.

All of the faces were also free of tattoos, piercings, or other markers. Importantly, the viewers -- described in the study as "perceivers" -- were shown an equal number of faces belonging to people who made over $150,000 a year (which the authors designated upper class) and under $35,000 a year (which the authors designated lower class).

The perceivers were asked -- based on the photos of faces alone -- who belonged to which group. Impressively, they were able to sort the faces into the correct categories 68 percent of the time, significantly higher than chance. "I didn't think the effects would be quite as strong, especially given how subtle the differences [in the faces] are," Rule says. "That's the most surprising part of the study to me."

Curious as to how the undergraduates were able to complete the task so successfully, Rule and Bjornsdottir next isolated specific facial features that might be cuing income status and conducted another test. Mouths were found to be the best wealth cues, although eyes were a strong indicator of income as well. What the authors eventually discovered, however, was that more significant than any specific part of the face was the apparent happiness of the face as a whole. Even the subtlest signs of positivity in a face, they concluded, were strong indicators of a higher social class.

"Once we figured out it was these subtle emotional expressions, it makes a lot of sense that the mouth would be showing most of that, because the mouth is what you mostly use, especially with positive emotions, like smiling," Rule says. The perceivers were also homing in on the eyes, because eyes, too, are strong indicators of happiness -- especially as indicators of repeated past happiness. "Crow's-feet around eyes refer to contractions of their reticular oculi muscles, which are activated when someone smiles," Rule explains.

In other words: If someone has had a generally happy life, he or she will look happy even while maintaining a supposedly "neutral" facial expression. And, while it's a broad generalization, people with higher incomes do tend to live happier and less stressful lives than those with low ones. Someone who takes the time to look at a face will -- at a rate far better than chance -- be able to see this repeated happiness and therefore judge that person as being of a higher social class.

To further prove this link, Rule and Bjornsdottir put photos of smiling faces in front of another group of volunteers. This time, faced with visibly happy faces rather than neutral ones, the perceivers weren't able to discern class any better than chance. Looking visibly happy, the researchers concluded, obstructs others' ability to discern your class, because it overrides the visible markers that appear in a more expressionless state.

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Although these findings are interesting in their own right, their relevance to the real world is even more important.

To demonstrate the vicious cycle that these kinds of snap judgments can perpetuate, Rule and Bjornsdottir asked yet another group of participants to evaluate faces and determine which ones would be likeliest to land an accounting job. (Earlier testing had shown that accounting was seen as a job that isn't particularly low- or high-class, which meant the perceivers wouldn't base their decisions on any preconceived notions about the role itself.)

As the researchers expected, the faces belonging to wealthier people were perceived as significantly more likely to land the job.

"We might be making a biased judgment based on something as subtle as these vestigial signals in someone's face," Rule says, "judging them on what their emotions have been like over the course of their lifetime." It would take a pretty gifted artist to depict something like that.