Few people knew that the country’s thirty-second President was paralyzed. Most knew that he’d had polio, but they remained unaware that he could not walk. Franklin Delano Roosevelt managed to hide the extent of his condition from the majority of the voting public with a simulated walking technique and a moratorium on photography of him in motion or in a wheelchair. His successor, Harry S. Truman, followed an opposite approach to publicity: for his first election campaign, he completed a train tour that covered some twenty-two thousand miles. At each stop, he would make sure that voters got a good, long look at him. Both Presidents lived before the era of televised debates and the constant presence of the media, but they had intuited the exact same thing: when it came to voter support, physical appearance mattered.

In 2003, the Princeton psychologist Alexander Todorov began to suspect that, except for those people who have hard-core political beliefs, the reasons we vote for particular candidates could have less to do with politics and more to do with basic cognitive processes—in particular, perception. When people are asked about their ideal leader, one of the single most important characteristics that they say they look for is competence—how qualified and capable a candidate is. Todorov wondered whether that judgment was made on the basis of intuitive responses to basic facial features rather than on any deep, rational calculus. It would make sense: in the past, extensive research) has shown just how quickly we form impressions of people’s character traits, even before we’ve had a conversation with them. That impression then colors whatever else we learn about them, from their hobbies to, presumably, their political abilities. In other words, when we think that we are making rational political judgments, we could be, in fact, judging someone at least partly based on a fleeting impression of his or her face.

Starting that fall, and through the following spring, Todorov showed pairs of portraits to roughly a thousand people, and asked them to rate the competence of each person. Unbeknownst to the test subjects, they were looking at candidates for the House and Senate in 2000, 2002, and 2004. In study after study, participants’ responses to the question of whether someone looked competent predicted actual election outcomes at a rate much higher than chance—from sixty-six to seventy-three per cent of the time. Even looking at the faces for as little as one second, Todorov found, yielded the exact same result: a snap judgment that generally identified the winners and losers. Todorov concluded that when we make what we think of as well-reasoned voting decisions, we are actually driven in part by our initial, instinctive reactions to candidates.

Todorov’s study indicated that election results might owe themselves somewhat to what the Nobel-winning psychologist Daniel Kahneman called fast, unthinking judgment, or what the psychologist Nalini Ambady calls thin-slice judgment: the ability to make any number of social judgments from a seconds-long experience. Students can predict a professor’s end-of-semester ratings from a silent video that lasts no more than ten seconds; employers can predict interview outcomes and hiring decisions from a little more; and voters can predict the results of elections from a judgment that is made in less than a second.

Obviously, many things affect voting decisions, from political platforms to sexting scandals. But if we control for the underlying factors, the research suggests that a thin-slice judgment retains its predictive validity, and it emerges as the single strongest predictor of victory beyond external factors such as broad economic data, like the unemployment rate; personal data, like age or gender; or any other single political measure, like whether someone is an incumbent or how much has been spent on the campaign. In one study of fifty-eight gubernatorial races, the only element that outperformed the thin-slice impression was a combination of two of the political factors most closely tied to electoral success: incumbency status and campaign spending. While we are never forced to vote based on one factor alone, the apparent predictive power of competence judgements reveal how deeply that quick impression may color our evaluation of more serious considerations.

The more data that Todorov gathered, the stronger his conclusion seemed. A few years after his initial research, he determined that even a one-second cutoff was unnecessary—the judgments that people made after a mere hundred milliseconds predicted election outcomes just as accurately as if they’d had an unlimited time to look at the photographs. But the more that his subjects reflected on their impressions—if they were asked explicitly to make a good judgment and think carefully about their choice—the less accurately their responses tracked with actual outcomes, and they became no different than a chance guess. Moreover, he found that judgments of trust and likability were not nearly as predictive as judgments of competence. It was competence alone that had the snap-judgment predictive power.

The findings have held in races for governor, as well as for the House and Senate. They have translated well outside of the U.S. political scene: competence ratings have predicted the results of elections in countries from Denmark to Bulgaria, and international participants have proven just at effective at predicting results in U.S. elections as Americans themselves. Competence ratings that were gathered more than a year before the 2008 U.S. Presidential primaries predicted with great accuracy who out of eleven potential Democratic and thirteen Republican nominees for President would go on to secure the actual nomination—and that’s before many were even officially being considered. That may be, in large part, because non-appearance-related factors are far more controlled in the nomination process: potential nominees have similar platforms within their parties, are operating in similar climates, and so on. Facial competence may then become a more powerful differentiating factor.

In a follow-up to his initial research, Todorov, along with the psychologist Nikolaas Oosterfhof, examined what features translated to the character judgments that he had observed earlier. Using computer analysis, the two determined that our rankings of faces came down to two principal components: valence, or trustworthiness, and dominance. The first tells us whether to approach or avoid someone, while the latter indicates if that person is physically strong or weak—and it is also the trait most closely tied to the appearance of competence. While the overall shape of the face made the greatest impression, certain markers like the nose, forehead, chin, eyebrows, and lips also translated reliably to increases on either dimension: baby-faced portraits—with softer faces, rounder chins, and higher foreheads—seemed more trustworthy. More masculine faces that were narrower, with more prominent chins and wider noses, seemed more dominant.

Those facial cues, in turn, may stem from a far more basic impulse, since we respond to those same features as children. In a 2009 study published in Science, the psychologists John Antonakis and Olaf Dalgas suggested that, when we judge a candidate as more or less competent, we do it in the same way that children do. They first asked a group of adults to rate pairs of faces, taken from the 2002 French parliamentary elections, based on how capable they seemed. When they compared the ratings to actual election results, the correspondence was seventy-two per cent. The ratings even predicted the margin of victory; the more competently-rated the face, the higher the margin. The researchers then had a group of children play a computer game, simulating a boat trip from Troy to Ithaca, in which they had to choose a captain for the voyage; their options consisted of the same 2002 election candidates. The two sets of responses were indistinguishable from each other: seventy-one per cent of the time, the children picked the election winner to pilot the boat.