Psychologists in the United States have discovered that voters tend to judge politicians with more immature features as less competent, and thus tend to favour opponents with a more grown-up appearance.

The findings suggest that Gordon Brown’s soft, round face may have less voter appeal than the more angular features of Tony Blair, and that the Conservatives might be better off picking a mature-looking leader such as Sir Malcolm Rifkind over David Cameron or David Davis.

They also offer a reason for the landslide defeat suffered by William Hague in the 2001 general election. Mr Hague boasts classic baby-faced features, and the Portuguese newspaper Diario de Noticias commented: “Never could a bald gnome with a baby face and monkey ears manage to defeat Blair.”

There are, of course, exceptions to the rule. It is hard to think of a more baby-faced politician than Sir Winston Churchill, and the craggy features of Senator John Kerry, defeated by President Bush last year, are anything but child-like.

The baby-face effect has been identified in two papers published today in the journal Science. The first study, led by Alexander Todorov, of Princeton University in New Jersey, examined US Senate races in 2000, 2002 and 2004, and US House of Representatives contests in 2002 and 2004.

Volunteers were asked to view pictures of the two leading candidates in each race, and asked to rate their faces for seven traits: competence, intelligence, leadership, honesty, trustworthiness, charisma and likeability. The results were ignored if either candidate was recognised.

The researchers found that scores for competence accurately predicted the results of the elections 70 per cent of the time, a much higher effect than would be expected by chance. None of the other factors had any measurable impact. In the second paper, Leslie Zebrowitz, of Brandeis University in Massachusetts, said that the results appeared to reflect the relative “baby-facedness” of the candidates.

Previous research has shown that people of any age who appear baby-faced, with a round face, large eyes, a small nose, a high forehead and a small chin, tend to be rated as less competent — though often as more trustworthy as well. “Although the study doesn’t tell us exactly what competence is — there are many kinds, including physical strength, social dominance and intellectual shrewdness.

Baby-faced people are perceived to be lacking in all these qualities,” Dr Zebrowitz said.

“The association between facial maturity and perceived competence is ubiquitous: baby-faced individuals within various demographic groups are perceived as less competent.”

The effect, she said, was also demonstrated by a study at Colgate University in New York state, in which scientists morphed the faces of the former US presidents Ronald Reagan and John F. Kennedy to make them appear more baby-faced. Volunteers found the altered images looked less strong, dominant and cunning.

Dr Zebrowitz, a psychologist and author of the book Reading Faces: Window to the Soul?, said that the evolutionary importance of detecting attributes such as emotion and character is probably responsible for the tendency to make snap judgments based on looks.

As the facial characteristics of babies signal vulnerability, people tend to generalise and assume that adults with similar features have similar qualities.

The effect applies across age, sex and race, although Dr Zebrowitz pointed out that women tend to have more child-like features that may place them at a disadvantage in politics.

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