In the new film Yes Man, Jim Carrey's character is convinced by a lifestyle guru that he needs to say yes to everything. He goes to a huge cultish rally, everyone is shouting “yes!” and he feels that he better join in. At first it works like a dream. But he ends up in hospital, begging to be released from the obligation. The guru is bemused. You're not under any obligation, he responds: “When I said that, I was riffing”.

I wonder if, a few months from now, Barack Obama will feel a little like that guru. Surrounded by enthusiastic people shouting “Yes we can”, he will patiently explain: “When I said that I was riffing.”

Mr Obama was elected on a wave of optimism, a surge of hope. But I think that when he is remembered by future generations it may be as the man who said: “No we can't.” Mr Obama's soaring rhetoric lifts hearts, his ambition excites the imagination, but just as impressive to those who meet him is his cool, detached demeanour. He may prove just the man America is looking for - the man who can unleash its “can't do” spirit.

The inaugural address yesterday was full of defiant language. The new President noted “a sapping of confidence across our land - a nagging fear that America's decline is inevitable, and that the next generation must lower its sights”. He said that this was a challenge and he promised to meet it.

Times Archive, 1933: Roosevelt: 'The only thing we have to fear is fear itself'

President Roosevelt assumed office on Saturday and delivered an inaugural speech remarkable for its candour and courage

• Mr. Roosevelt's speech

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• 1861: President Lincoln's inaugural speech

Then he added: “Now, there are some who question the scale of our ambitions - who suggest that our system cannot tolerate too many big plans. Their memories are short. For they have forgotten what this country has already done; what free men and women can achieve when imagination is joined to common purpose, and necessity to courage.”

Yet accompanying these bullish assertions, delivered as they were (it has to be said) with stunning force (almost too great for the words to bear), was a downbeat message. He described a nation at war, an economy badly weakened, a collective failure to make hard choices. And while the new President promised to face these difficulties, he was extremely careful not to promise to eliminate them.

His attempt to summarise his attitude to the state produced this: “The question we ask today is not whether our Government is too big or too small, but whether it works.” In other words, he offered his audience ruthless pragmatism, a clear-eyed realism distinct from the upbeat message of the campaign trail.

On foreign policy, too, his message was one of restraint. In a speech that was longer on poetic phrases than on arresting ideas, one of the most interesting passages was when he said that earlier generations “knew that our power grows through its prudent use” and that security emanates from “the tempering qualities of humility and restraint”.

Mr Obama's inaugural address, then, was a piece of expectation management. Already he is warning his supporters to understand the limits of change and the constraints he is under.

It was a distinctly different tone from the campaign. But then it is not hard to see why many election campaigns (Mr Obama's was routine in this respect) are relentlessly optimistic. Optimism works at the ballot box. “Yes we can” is the ideal slogan for the hustings.

In the classic text Learned Optimism, Professor Martin Seligman explains the great advantages that come from an optimistic style of explaining events to yourself. (Do you think the good things that happen to you are permanent, a symbol of your general good fortune and talent, and a result of your own action rather than that of others? If so, you are an optimist.) Optimists are more successful at school, rise higher at work, they live longer, healthier lives and, naturally, they are much less prone to depressive illness.

When Seligman worked as a consultant for the Metropolitan Life insurance company, he helped the firm to improve its sales significantly by screening job applicants for their explanatory style. Optimists picked themselves up after bad sales calls and went right on calling. Pessimists gave up. A study of baseball teams suggested that those who explained their victories and defeats optimistically were more successful than those whose explanations were pessimistic.

But in no field is optimism more potent than in politics. With his colleague Harold Zullow, Seligman embarked on a study of the public statements of presidential candidates between 1900 and 1988. Were they “Yes we can” optimists? Or did they ruminate on difficulties, blaming themselves, seeing setbacks as permanent? Analysing their campaign speeches, blind to the person delivering them, the psychologists assigned them a score for pessimism. In election after election, the more optimistic candidate won.

There were exceptions - Richard Nixon defeated the “happy warrior” Hubert Humphrey and, rather surprisingly, Franklin Roosevelt was more pessimistic than any of his four defeated rivals. Yet optimism proves to be a brilliant predictor of victory and victory margin.

So if optimism is such a huge advantage in life, why are there pessimists at all? What evolutionary advantage does pessimism provide? Why does it continue to exist? Seligman's conclusion is sobering: “Depressed people see reality correctly, while nondepressed people distort reality in a self-serving way.”

The evidence of numerous studies suggests “that depressed people, though sadder, are wiser”. Take, for instance, a study in which subjects were given varying degrees of control over the lighting in a room. They all pressed the switch, but the switch only worked for some subjects. Optimists hugely overestimated the control they were exerting. Pessimists estimated their degree of control correctly.

So optimists need pessimists if they are not to become entirely detached from reality. And I believe that for all his “Yes we can” rhetoric, history may cast Mr Obama as just such a restraining influence. The Dreams from My Father of his memoirs were not the thunderous proclamations of Martin Luther King's dreams. They were quiet, somewhat pessimistic, ruminations on identity and belonging.

He may be the President who contains America's overseas adventures to ensure that what it starts, it can finish, who manages its transition from sole superpower to the sharing of world leadership, who understands the constraints imposed on the state and public purse by the colossal financial crisis that is engulfing us, who prefers moderation to the enumeration of a new doctrine.

Mr Obama's election has widely been seen as ushering in a new era of ambition and optimism. Prepare for quite the opposite.

daniel.finkelstein@thetimes.co.uk

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