Happiness is Slough in your rear mirror, said a car sticker, briefly famous in the 1980s. Betjeman called on friendly bombs to fall on the place. Now, it seems, Slough has become a byword for misery - “a treasure trove of unhappiness, buried beneath a copious layer of gloom”, according to a former New York Times journalist Eric Weiner, who has been travelling the world in search of happy places, and has lit on Slough as one of the saddest.

I don't know what Slough has done to deserve all this, except that its name is against it. Slough, as in slough of despond, I suppose, or the word itself, meaning a hollow filled with mud, or (so my dictionary tells me) the dead tissue in a sore; or perhaps it's just that Ricky Gervais set his skin-crawling comedy series The Office in it. Someone should had thought of all that before they named the place.

There is nothing much to be said for the generalisations to which instant observers like Mr Weiner are prone. Nor for the idea, claimed last week by a research team from the University of Edinburgh, that happiness lies in our genetic structure, thus fating us from birth to be happy or grumpy; we either have an “affective reserve” of contentment that we can call on in times of stress, they say - or we do not. By that token the British, with their shared gene pool, are, in Mr Weiner's terms, a wretched nation, with Slough the capital of gloom. “I feel sorry for the Brits,” he writes. “They don't merely enjoy misery, they get off on it.”

Bunkum. We get out of bed happy one day, wretched the next, conditioned as much by the fact that the plumber failed to turn up as by the war in Afghanistan, or the return of I'm a Celebrity... Get Me Out of Here. Emotions like happiness are far too complex and variable to conform to the lazy notion that we are defined by our genes - that a jolly or a melancholy chromosome can be identified, and then, presumably, modified at birth to guarantee a lifetime of Falstaffian bonhomie.

I exclude the depressives, the bipolar victims, and those who are heir to generations of mental deficiencies that govern their lives and those who are closest to them. For the rest of us, nature, not nurture, is the governing influence. Take Gordon Brown, take David Cameron. The Edinburgh team would doubtless argue that Mr Brown was handed the grumpy gene at birth, that this dour and introspective character, so cautious in his dealings with others, so ready to take offence, obsessed by detail and unable to relax in company, was one of nature's pessimists, a man for whom no silver lining comes without a cloud, the epitome of P.G.Wodehouse's famous remark that it is never difficult to distinguish between a Scotsman with a grievance and a ray of sunshine.

Those who remember the Gordon Brown of old can summon up a very different figure - the scintillating radical at Edinburgh university, the life and soul of political dinner parties when John Smith was leader, the romantic who could charm a roomful of people, the father who could weep openly over the loss of his first child and engage in passionate debate. Did that man disappear altogether, or did the iron enter his soul when he lost the battle for the leadership with Tony Blair and dedicated himself to reclaiming his inheritance, with everything subjugated to that end?

David Cameron, by contrast, would seem to have inherited the charm gene, the equable, laidback, arm-around-the-shoulder gregariousness that gives him an advantage at the dispatch box and a good ten points in the opinion polls. Surely he, of all people, was dealt one of nature's more generous hands, an ability to engage with the public and see off his critics without resentment? To which the answer is - it's a little too early to say. Those who know him well say that he possesses (don't we all?) a streak of irritability that can surface when things are not going his way. Faced with a significant rebellion or the collapse of his economic policy, and then let's see whether he can maintain that languid posture at the dispatch box, the infectious grin, the self-deprecating shrug that keeps stress at bay.

Scratch the surface of the most genial cove you know, and beneath it is likely to lie a darker side; the finest comedians are prone to gloom (cue Tony Hancock, Frankie Howerd, Jack Dee); there is none so miserable as the clown. I was struck, reading Valerie Grove's enthralling biography of John Mortimer - surely the epitome of the happy man - by how determinedly he keeps his own self-doubt at bay, and how a carapace of jollity has protected him from the unhappiness he has often created in others. “The Falstaffian exterior made him immune to guilt and insecurity,” Grove wrote. Is Sir Richard Branson, with that permanently fixed smile, a naturally happy man, or a ruthless monomaniac, whose stock in trade is the projection of infectious enthusiasm? Is Barack Obama as imbued with the same feel-good factor as Ronald Reagan or Dwight D. Eisenhower, or will the worm turn when the going gets rough?

What is undoubtedly true is that the world is divided between those who project happiness and those whose personal struggles are all too evident to others.

Contrast the saturnine Mr Brown with the cheerful SNP leader Alex Salmond in Scotland, who sweeps all before him and conveys an air of easy confidence to the nation. One has suppressed his better side, the other makes good use of it.

Perhaps, in the end, that is as close as we are likely to get to a definition - that he who makes others happy becomes in the process a happier person. Either way, I doubt if it supports the gene theory. We are born, Janus-like, with both tendencies. It is up to us which one we choose to cultivate.

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