Harriet Walker: The long and the short of it – a haircut can sum you up

What's dull, thick, smooth and coarse and is currently on everyone's minds? No, not politicians or pop stars – something much more literal than that: the answer is hair. We're a nation obsessed in our pursuit of the hirsute.

From the keening over actress Emma Watson's decision to cut all of hers off, to our commoners' awe at Kate Middleton's abundant and flowing locks, our fascination goes far beyond the follicle. We characterise other people by their coiffures; we make assumptions about them. Stringy and lank? Someone to avoid sitting next to on the bus. Shiny and full? Ripe for a career in TV. See? It's easy! Let's try another.

Julian Assange may be the most reviled and revered man in the world right now; he may even be a threat to national security, but that hasn't stopped us going after his short and curlies. And his elaborate silver quiff sums him right up: a greying faux intellectual sexed up with a bit of styling product and turned into an anti-hero. He even had it cut for his TV appearance after being released on bail (to Bungay, Suffolk, which by all accounts is a town with an unusually large number of hairdressing salons, no less). Assange knows the power of a pompadour as well as any popstar does, and this reveals more about him than any press conference ever will. There are more subtexts here than in a leaked cable.

Other official documents laid bare to the public this week include Brylcreem's Best Celebrity Hair 2010 poll to celebrate the 9th annual National Men’s Hair Week. What a diplomatic kerfuffle when Simon Cowell finds out he's in the bottom 10, while his protégé Harry Styles (he of the Byronic mop) has made the top spot. How will Prince William feel when he finds out the public has chosen his brother's carrot-top over his own thinning thatch?

Now, I'd be the first to point out that a Best Celebrity Hair 2010 poll does not a zeitgeist make, but it does point to a certain localised scrutiny – a Best Celebrity Nose contest is a rarer sight, I'm sure you'll agree, or Best Celebrity Forefinger. But hair is an emotive subject all round – just ask people who are losing theirs, or those who hide its allure beneath a hijab. Ask the punks who chose the Mohican to look as anti-establishment as possible, or the French collaborators who had their heads publicly shaved. When David Cameron changed the side he wears his parting on, he made more of a political U-turn than his buddy Nick Clegg.

We're attached to our hair; its roots go deep. But it's also a great leveller. You think William chose Kate because she's a nice girl? Think again. It's because she has rich girl's hair, despite her humble origins. You can't achieve those sleek tresses; you're either born with them or not. Kate's hair will reinvigorate the royals like a zesty shampoo.

Disney last weekend released its latest cartoon, Tangled, a reworking of that ultimate tonsorial archetype Rapunzel. Meanwhile, pint-sized popstar Willow Smith (who last week reached No 2 in the charts with "Whip My Hair") has had her trademark plaits cut into a bowl cut. And the now gaminely cropped Emma Watson told of her relief to be free of the Harry Potter contract that stipulated she did not cut her hair for 10 years. Hairdressers speak of traumatised break-up victims begging for drastic chops; our hair is an expression of our inner turmoil. Just ask Boris Johnson.