Charles Darwin is famous for his prolific writing about biology. In addition to publishing his theory of evolution, Darwin wrote books about coral reefs, earthworms and carnivorous plants. But the eminent naturalist made important contributions to more than just the life sciences. It turns out Darwin was also an early experimental psychologist.

Darwin conducted one of the first studies on how people recognize emotion in faces, according to new archival research by Peter Snyder, a neuroscientist at Brown University. Snyder's findings rely on biographical documents never before published; they now appear in the May issue of the Journal of the History of the Neurosciences.

While looking through Darwin's letters at the University of Cambridge in England, Snyder noticed multiple references to a small experiment on emotion that Darwin had performed in his house. With the help of librarians, Snyder uncovered the relevant documents—research notes and tables filled with the illegible scrawl of Darwin's elderly hands and the neater writing of his wife Emma. Although Darwin's fascination with emotional expression is well documented, no one had pieced together the details of his home experiment. Now, a fuller narrative emerges.

"Darwin applied an experimental method that at the time was pretty rare in Victorian England," Snyder said. "He pushed boundaries in all sorts of biological sciences, but what isn't as well known are his contributions to psychology."

In 1872, Darwin published The Expression of the Emotions in Man and Animals, in which he argued that all humans, and even other animals, show emotion through remarkably similar behaviors. For Darwin, emotion had an evolutionary history that could be traced across cultures and species—an unpopular view at the time. Today, many psychologists agree that certain emotions are universal to all humans, regardless of culture: anger, fear, surprise, disgust, happiness and sadness.

In writing Expression, Darwin corresponded with numerous researchers, including French physician Guillaume-Benjamin-Amand Duchenne, who believed that human faces expressed at least 60 discrete emotions, each of which depended on its own dedicated group of facial muscles. In contrast, Darwin thought the facial muscles worked together to create a core set of just a few emotions.

Duchenne studied emotion by applying electrical current to the faces of his subjects, sending their muscles into a state of continual contraction. By stimulating the right combination of facial muscles, Duchenne mimicked genuine emotional expression. He produced more than 60 photographic plates of his subjects demonstrating what he believed were distinct emotions.

But Darwin disagreed. "I started to look at the actual folio of photographic slides that Darwin had received from Duchenne," Snyder said. "And Darwin wrote these faint notes on it saying, 'I don't believe this. This isn't true.'"

Darwin hypothesized that only some of Duchenne's slides represented universal human emotions. To test this idea, he arranged a single-blind study at his home in Kent County, England. Darwin chose 11 of Duchenne's slides, placed them in a random order and presented them one at a time to over 20 of his guests without any hints or leading questions. He then asked his friends to guess which emotion each slide represented and tabulated their answers. That kind of experimental control would be considered minimal today, but it was progressive for Darwin's time, Snyder pointed out.

According to the handwritten notes and data tables Snyder found, Darwin's guests agreed almost unanimously about certain emotions—like happiness, sadness, fear and surprise—but strongly disagreed about what other more ambiguous slides showed. For Darwin, only photographic slides that earned overwhelming agreement depicted one of the true universal human emotions. The others were just Duchenne's failed simulations.

Darwin used the results of his 19th-century experiment to inform his own understanding of emotion and his writing of Expression. But his pioneering methods remain relevant to psychologists today. Snyder and his co-authors write that Darwin's little-known experiment is a forerunner of modern psychology experiments on people who cannot properly recognize emotion in faces.

"Today, we use almost the same technique, and even stimuli, to evaluate emotional recognition in a variety of psychiatric diseases, like autism and schizophrenia," Snyder said. "Darwin's method and approach are not locked in time."

Images of Guillaume-Benjamin-Amand Duchenne stimulating a subject and Duchenne’s photographic slides courtesy of Wikimedia Commons