For each of the three races, the researchers arranged for half the subjects to see a flier in which the candidate with the more favorable appearance was pictured as the liberal Democrat, while the other half saw him pictured as the conservative Republican. That way, if looks didn’t matter, the two candidates should receive about an equal number of votes (regardless of the split in party preference among the participants). Instead, the voting split about 60-40, with a majority favoring the candidate with the better visage.

A related series of studies, also led by Professor Rosenberg, showed that candidates could exert some control over the appearance factor. Researchers first recruited 210 volunteers to rate head-and-shoulder shots of hundreds of women in terms of how “able looking” they were. From these ratings they determined that certain factors contributed to this appearance: for example, eyes with more curvature on the top than the bottom; hair that is short and parted on the side or combed back; a hairline that comes to a slight widow’s peak; a broad or round face; and a smile. Then they employed a Hollywood-style makeup artist and a photographer to use these criteria to create two images of each candidate, one more able looking and one less. (A second study confirmed that the manipulations had the desired effect.)

Finally, the researchers recruited another set of volunteers to do the voting. Each candidate was presented in her “attractive” form to half the subjects, while her opponent was presented in her “unattractive” form. The other half of the subjects saw the same women running under the same party banners, but with the appearance variable reversed. On average, the candidates received 56 percent of the vote when portrayed by the better campaign photo, compared with 44 percent when portrayed by the unfavorable photograph — a vote swing of 12 percentage points.

In another series of studies, conducted at Princeton by the psychologists Alexander Todorov and Charles C. Ballew II, participants were presented with pairs of head-shot photos of the competing candidates in hundreds of actual Congressional and gubernatorial elections in the United States. After displaying a photo pair for just a quarter of a second, the researchers asked the participants to judge which candidate was more competent. (If a participant recognized a candidate, his response for that race was not counted.) These fleeting and uninformed impressions of competence turned out to correlate strongly with the actual election results. Over the hundreds of races tested, the more competent-looking candidate won the real-world election about 70 percent of the time.