After Supreme Court Justice Oliver Wendell Holmes was introduced to Franklin D. Roosevelt in the 1930s, he famously described the president as having a "second-class intellect, but a first-class temperament." Most historians would agree; Roosevelt's success as a leader rested more on his emotional intelligence than his analytical IQ.

Contrary to the view that emotions always interfere with thinking, the concept of emotional intelligence holds that the ability to understand and regulate emotions can make overall thinking and performance more effective. Leaders who have this self-awareness, discipline and empathic capacity are better able to channel their personal passions and attract supporters.

Psychologists have wrestled with ways of measuring intelligence for more than a century. General IQ tests assess certain dimensions, such as verbal and spatial dexterity, but IQ scores predict only 10 percent to 20 percent of variation in academic or occupational success. The other 80 percent is the product of hundreds of variables playing out over time, of which emotional intelligence is just one. But experts concur that it is an important, learnable component of leadership that increases with experience and that different people possess it in different degrees.

Emotional intelligence helps leaders color their charisma to suit shifting contexts. We all change how we present ourselves to control the impression we make. Everyone has heard the expression "dress for success" and seen how politicians swap their silk ties for flannel shirts while campaigning in farm states. Even a tough general like George Patton used to practice his scowl in front of a mirror. Robert Mugabe, the brutal president of Zimbabwe, switches from Shona to English to send different messages to different audiences.

You can't fake emotional intelligence, but it does require some of the same skill possessed by good actors. Ronald Reagan's screen experience served him well in this regard, and Roosevelt was a master "actor." Despite his pain and difficulty in moving because of polio, he maintained a smiling exterior and was careful about how he was photographed. Critics sometimes fault the Barack Obama or John McCain campaigns for trying to stage-manage their candidates' appearances, but this is nothing new. It has simply gotten much more difficult because unmanaged moments can so easily find their way to YouTube or the blogosphere.

Effective leadership also requires careful attention to one's moods, which can initiate an emotional chain-reaction office-wide as everyone takes his cue from the boss. Closely watched CEOs and presidents are always conveying signals; the emotionally intelligent ones are aware of that and control them. They also exercise self-discipline that prevents their personal psychological needs from taking over. Richard Nixon was weak on emotional intelligence. He was able to strategize effectively on foreign policy but couldn't manage the personal insecurities that eventually led to his downfall.

George W. Bush showed emotional intelligence in overcoming his alcohol problem and in displaying conviction in policies even if they were politically unpopular. But at some point, his perseverance became stubbornness, impeding his ability to learn, adjust and be effective.

As we pick a new president this year, we are doing more than sizing up policy positions -- we are seeking clues to how Obama and McCain compare on emotional intelligence. We are picking apart their personal biographies, examining their mannerisms, studying how they choose their words and hold their emotions in check (or not).

So far, both have shown aptitude. Consider the way McCain revived his moribund campaign a year ago, or how Obama turned the embarrassment of the Rev. Jeremiah A. Wright Jr.'s views into an opportunity to give a first-rate speech on race in America. But both will face more emotional IQ tests in the tough months ahead, and the voters will be watching and scoring.

Joseph S. Nye Jr. is a professor at Harvard University.

(July 17, 2008)