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.
No way to control for grooming, though. In a world with $400 face cream, grooming needs to be controlled for.
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.
I'd be curious how they did this.