Queen Elizabeth’s recent US visit caused a certain sartorial unease in Washington. The Senate Democratic leader, Harry Reid, for example, declined an invitation to the State dinner in part for fashion reasons. As his spokesperson told The New York Times, he “isn’t much of a white-tail-at-dinner kind of guy”.

It was a pretty blunt statement, with a fair amount of subtext. The meaning, however, seemed clear: her Majesty was then, this is now, and Reid is a modern kind of guy. He’s not interested in the old empire – or, for that matter, their old dress codes.

He’s not the only one. Even though a recent group photo of the US Republican presidential candidates might have featured an indistinguishable array of nine men in navy two-button suits (plus one man in a navy three-button suit), the new generation of wannabe western world leaders seems to have decided, to a man, that the best way to put substance behind their assertion of “change” is to change their clothes.

Consider Barack Obama, who launched his presidential campaign on the internet without a tie and featured on the cover of Men’s Vogue in similarly dress-down garb (indeed, Obama seems to be developing this into a bit of a signature). Or take the new French president Nicolas Sarkozy, who abandoned his usual glossy striped or dotted tie in favour of an open collar for his first appearance after winning the national election. And then there is the UK Conservative party leader David Cameron, who, with his snazzy Converse sneakers, is busy drawing a line in the sand between himself and Gordon Brown, the man certain to be Britain’s next prime minister.

Fashion has always been one of the great weapons and expressions of generational change – so much so that it’s practically an eye-rolling cliché. Every family experiences this in microcosm (c’mon, tell me the story of your declaration of sartorial independence – mine involved second-hand men’s overcoats and lots of black nail polish). Indeed, when I was shopping around for schools for my children, and finally found one I liked, I was brought up short by their lack of uniform. “But why?” I wailed (I like the idea of a uniform – it makes shopping easier and avoids the scary Britney Spears midriff problem).

“We think clothing is the safest form of self-expression,” was the response. Given the alternatives, I could accept this – though I’m sure there are those, like the late Rev Jerry Falwell, the US televangelist, who feel that self-expression through clothing is a slippery slope that leads to self-expression through crystal meth. But that’s another debate.

Anyway, the point is that there’s no reason politicians should be exempt from this sort of semiotics. And yet such change in dress is frequently regarded as suspicious by a large portion of the voting public, who see the tie-or-no-tie stance as yet another attempt at insincere manipulation (these are also the people who read Cameron’s change in hair parting from left to right as some sort of political subterfuge).

Admittedly, to a certain extent this is true, in that all presentation of all people is a form of manipulation of the viewer – an attempt to convince them you are trustworthy or artistic or radical or intelligent. Witness Julia Roberts in glasses testifying before Congress in 2002. The question is rather whether the attempt has integrity.

And in that context, though I have no way of assessing the dress choices of Sarkozy or Obama, I can say that if Cameron’s wife is anything to go by, his sports jacket-and-chinos thing may be genuine.

In the normal course of my life, I don’t tend to run into party leaders’ spouses but it happens that Samantha Cameron is also the creative director of Smythson, the stationery and leather goods house. Recently, while I was at Smythson inspecting some new handbags, she was there too (their big autumn/winter number is a soft quilted squishy thing that looks like a contemporary grandmother’s purse). She was wearing scuffed black sneakers, black jeans and a black car coat. She had been riding her scooter over from the company’s workshops in the East End and had dumped her helmet and bag in a corner.

Generally, people in the fashion world feel a compunction to dress up in some way that represents the values they want associated with their brand, much in the way politicians do. And, indeed, Mrs Cameron’s choices did reflect pretty closely the whole Smythson thing: well-made, not showy and not trendy. Call me a sucker, but there was a consistency to her self-presentation that seemed relatively unassumed.

Now, guess who buys the clothes in that family?

vanessa.friedman@ft.com

More columns at www.ft.com/friedman