WASHINGTON — While this year’s presidential campaign has been marked by historic firsts, the nominations of senators McCain and Obama will renew one surprising trend: For the fifth time in the last 35 years, America will have a lefty in the White House.

Both major party candidates are southpaws, contributing to a largely unexplained phenomenon that has vexed researchers and historians — and drawn notice from a federal judge destined for the Supreme Court. Though left-handers comprise just 10% of the population, they are dominating presidential politics.

Their recent success transcends ideology. Since 1974, presidents Ford, Reagan, George H.W. Bush, and Clinton have all favored their left hands, while President Carter and the current President Bush are righties. The trait is also not exclusive to winning candidates: Vice President Gore is left-handed, as are past presidential contenders Robert Dole, John Edwards, Bill Bradley, and Ross Perot. A prominent New Yorker who flirted with a White House bid, Mayor Bloomberg, is a lefty.

Researchers who have studied handedness have found links to genetics and to brain function, but there is no prevailing theory to explain the plethora of left-handed commanders in chief in recent decades.

Yet the trend is more than a statistical anomaly, a professor of neurology and psychiatry at the University of California Los Angeles School of Medicine, Daniel Geschwind, said.

“It’s definitely not an accident. The chance is less than one in a thousand,” he said.

Before 1974, just two presidents were known definitively to be left-handed: James Garfield and Harry Truman.

Studies have shown that whereas righties favor the left hemisphere of their brain, which controls language, left-handers are more likely to have bilateral brain function, which could allow them to visualize problems more broadly and with more complexity. A higher percentage of mathematicians and scientists are left-handed, and the same is true for artists.

Bilateral brain function could relate to the social and interactive skills needed to be successful in politics, but not enough research has been done, Dr. Geschwind said.

Left-handedness also has proved a distinct advantage in certain sports, including tennis and baseball, where southpaws are prized both in pitching and batting.

At the same time, studies have suggested an increased prevalence of schizophrenia, autism, and bipolar disorder among left-handers.

Twins are more likely to be left-handed, research shows, and handedness has tended to run in families, although that was not the case for the elder Mr. Bush, a lefty, and his son George W., a righty.

A scientist at the National Cancer Institute, Amar Klar, has found another, more novel trait that may distinguish left-handers from right-handers: hair growth. “Handedness is related to the way the hair spins on the back of your head,” he said in an interview.

His research shows that the whorl for right-handers curls clockwise in 92% of cases. In left-handers, the distribution is random, with half exhibiting a clockwise whorl and the other half spinning counterclockwise. Mr. Klar said he could spot a counterclockwise whorl from seeing Mr. McCain and Mr. Clinton on television and looking at the way they appear to comb their hair.

Researchers have long debated why left-handers have succeeded as a distinct minority group in a society dominated by right-handers. “People like to think there’s something wrong with left-handers,” a professor of psychology at University College London who has written a book on handedness, Christopher McManus, said.

Yet the percentage of left-handed people across the population has remained stable at between 8% and 10%, a statistic that stands in contrast to other animal species and argues in favor of certain advantages to being left-handed. “Something is keeping 10% of the population there,” he said.

One theory to explain the success of left-handers in politics is that, at an early age, they recognize that they are different in a fundamental way from most of their peers, said Melissa Roth, the author of “The Left Stuff: How the Left-Handed Have Survived and Thrived in a Right Handed World.”

“Their difference might be treated as a positive or a negative, a ‘creative’ asset or a failure to adapt, but either way they are aware that they are ‘special,’ and that’s a trait psychologists find in many leaders,” Ms. Roth said.

Still, some contend that the phenomenon of left-handed presidents is no more than a blip. An associate justice of the Supreme Court, Samuel Alito, even offered that argument — and drew criticism for doing so — in dissenting from a ruling that overturned a murder conviction of a black man who had been tried before an all-white jury. Justice Alito, then a member of the 3rd U.S. Circuit Court of Appeals, took issue with a majority opinion that said “an amateur with a calculator” could have figured out the appropriate percentage of black jurors in the county where the case was tried.

“Although only about 10 percent of the population is left-handed, left-handers have won five of the last six presidential elections,” Judge Alito wrote. “Our ‘amateur with a calculator’ would conclude that ‘there is little chance of randomly selecting’ left-handers in five out of six presidential elections. But does it follow that the voters cast their ballots based on whether a candidate was right- or left-handed?”

The judge did not mention that in at least two of those elections — 1992 and 1996 — the voters did not have much of a choice: As in the coming election, the leading candidates were all left-handed.