Last month, 50 former national security officials who had served at high levels in Republican administrations from Richard Nixon to George W Bush published a letter saying that they would not vote for their party's presidential nominee, Donald Trump.

In their words, "a president must be disciplined, control emotions, and act only after reflection and careful deliberation". Simply put, "Trump lacks the temperament to be president".

In the terminology of modern leadership theory, Trump is deficient in emotional intelligence - the self-mastery, discipline and empathic capacity that allows leaders to channel their personal passions and attract others.

Contrary to the view that feelings interfere with thinking, emotional intelligence - which includes two major components, mastery of the self and outreach to others - suggests that the ability to understand and regulate emotions can make overall thinking more effective.

EQ over IQ?

While the concept is modern, the idea is not new. Practical people have long understood its importance in leadership.

In the 1930s, former Supreme Court Justice Oliver Wendell Holmes, a crusty old veteran of the American Civil War, was taken to meet Franklin D Roosevelt, a fellow Harvard graduate but one who had not been a distinguished student.

Asked later about his impressions of the new president, Holmes famously quipped: "Second-class intellect; first-class temperament". Most historians would agree that Roosevelt's success as a leader rested more on his emotional than his analytical IQ.

Psychologists have tried to measure intelligence for more than a century. General IQ tests measure such dimensions of intelligence as verbal comprehension and perceptual reasoning, but IQ scores predict only about 10-20 percent of variation in life success.

Trump's narcissism has led him to overreact, often counter-productively, to criticism and affronts.

The 80 percent that remains unexplained is the product of hundreds of factors playing out over time. Emotional intelligence is one of them.

Some experts argue that emotional intelligence is twice as important as technical or cognitive skills. Others suggest it plays a more modest role.

Moreover, psychologists differ about how the two dimensions of emotional intelligence - self-control and empathy - relate to each other.

Bill Clinton, for example, scored low on the first but high on the second. Nonetheless, they agree that emotional intelligence is an important component of leadership. Richard Nixon probably had a higher IQ than Roosevelt, but much lower emotional intelligence.

Awareness and control of signals

Leaders use emotional intelligence to manage their "charisma", or personal magnetism, across changing contexts.

We all present ourselves to others in a variety of ways in order to manage the impressions we make: For example, we "dress for success". Politicians, too, "dress" differently for different audiences.

Ronald Reagan's staff was famous for its effectiveness in managing impressions. Even a tough general like George Patton used to practise his scowl in front of a mirror.

Successful management of personal impressions requires some of the same emotional discipline and skill possessed by good actors.

Acting and leadership have a great deal in common. Both combine self-control with the ability to project.

Reagan's prior experience as a Hollywood actor served him well in this regard, and Roosevelt was a consummate actor as well.

Despite his pain and difficulty in moving on his polio-crippled legs, Roosevelt maintained a smiling exterior, and was careful to avoid being photographed in the wheelchair he used.

Humans, like other primate groups, focus their attention on the leader. Whether chief executives and presidents realise it or not, the signals they convey are always closely watched.

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Emotional intelligence involves awareness and control of such signals, and the self-discipline that prevents personal psychological needs from distorting policy.

Nixon, for example, could strategise effectively on foreign policy; but he was less able to manage the personal insecurities that caused him to create an "enemies list" and eventually led to his downfall.

The case with Trump

Donald Trump has some of the skills of emotional intelligence. He is an actor whose experience hosting a reality-television show enabled him to dominate the crowded Republican primary field and attract considerable media attention.

Dressing for the occasion in his signature red baseball cap with the slogan "Make America great again", he appeared to have gamed the system with a winning strategy of using "politically incorrect" statements to focus attention on himself and gain enormous free publicity.

But Trump has proved deficient in terms of self-control, leaving him unable to move towards the centre for the general election.

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Likewise, he has failed to display the discipline needed to master the details of foreign policy, with the result that, unlike Nixon, he comes across as naive about world affairs.

Trump has a reputation as a bully in interactions with peers, but that is not bad per se. As the Stanford psychologist Roderick Kramer has pointed out, President Lyndon Johnson was a bully, and many Silicon Valley entrepreneurs have a bullying style. But Kramer calls such figures bullies with a vision that inspires others to want to follow them.

And Trump's narcissism has led him to overreact, often counter-productively, to criticism and affronts. For example, he became embroiled in a dispute with an American Muslim couple whose son, a US soldier, was killed in Iraq, and in a petty feud with Paul Ryan, the Speaker of the House of Representatives, after Trump felt slighted. In such cases, Trump stepped on his own message.

It is this deficiency in his emotional intelligence that has cost Trump the support of some of the most distinguished foreign policy experts in his party and in the country.

In their words, "he is unable or unwilling to separate truth from falsehood. He does not encourage conflicting views. He lacks self-control and acts impetuously. He cannot tolerate criticism."

Or, as Holmes might say, Trump has been disqualified by his second-class temperament.

Joseph S Nye, a former US assistant secretary of defence, is a professor at Harvard University and the author of Is the American Century Over?

The views expressed in this article are the author's own and do not necessarily reflect Al Jazeera's editorial policy.

Copyright: Project Syndicate 2016 - Trump's Emotional Intelligence Deficit