There is a moment when, in our heads, the deal is done. We decide to buy. We may have been searching for a particular piece of clothing for months, or it may be love at first sight. Either way, we’re heading for the till and reaching for our wallets. But what made us want this object in the first place? Who or what really influences what we wear?

Trying to pin influence down is like looking for hay in a haystack: we know it’s there, but the separate pieces are often too many to count and, on their own, insignificant. Together, though, they make the synapses fire along the neural pathway that leads to Pay Now, and if there are enough of them, making regular appearances, they add up to our personal style.

So why do we fall for a certain garment and feel nothing at all for its neighbour on the rail? The Style editor asked me to put a single purchase under the microscope. I’ll take a recent amour, a green-grey military-style jacket from the French-owned chain Zadig & Voltaire. It has quite a few pockets, a bit of gold piping around the collar and cuffs, and a fake medal on the right breast. What was it that drew me, with what felt like inevitability, towards it?

Well, first of all, it ticked some practical boxes — I know the colour suits me, it’s a mid-thigh length I like, it’s made of natural fibres and it was the right weight for summer. Also, military has been having another of its Fashion Moments. And maybe I’m attracted by its French pedigree — it must be chic if it’s French, right?

Then there’s the matter of its maleness. Let’s not go too deep here, but I’ve always had a penchant for theatrical, principal-boy-type jackets. I still mourn a Selina Blow velvet frock-coat that disappeared when my flat was burgled 20 years ago, and I suspect its ghost haunts my latest acquisition. There’s also the matter of militariness: I am the least likely candidate for a career in the army and go light-headed at the sight of blood, but perhaps I enjoy the tension between appearance and reality. Or perhaps it makes me feel tougher than I really am?

There is the wider cultural context in which a military jacket sits, immortalised not just in war photography but in painting: Goya’s portrait of Wellington or Ingres’ of Napoleon, both looking splendid in their medals. Eventually such jackets were used counter-culturally, with the Beatles appearing in garish frock coats complete with gold frogging and epaulettes on the “Sgt Pepper” album cover. David Bowie and Freddie Mercury knew how to subvert the military look, too. All these associations and more are layered like a millefeuille in my consciousness. If I dig even deeper, I unearth my brother’s toy soldiers, and then a magical, childhood trip to the ballet when I was far more smitten with the crisp lines of the Nutcracker’s red-and-gold jacket than by the floaty, girly tutus.

Each item in our wardrobes has a swirling hinterland of Proustian associations and allusions that make the difference between something feeling “me” and “not me”. A white t-shirt may have a simpler back-story than my jacket, but it will still be sparking its own connections in the brain: James Dean, summer, will look good with a tan, even, perhaps, school gym-kit…Our wardrobes contain so much more than clothes. To say that we express ourselves through what we wear makes it sound far too deliberate and articulate a process. We express ourselves through our clothes in the same way that we express ourselves through the way we move our bodies or gesture with our hands. The impulses behind each movement are a subtle mix of the learned and the inherited, the conscious and the unconscious, the imagined and the real, the individual and the cultural.

And I have not even told you the whole truth about my jacket. When I first fell for it, a couple of years ago, they had run out of my size — and anyway, I thought it was overpriced. Two years later, browsing on eBay in an idle moment, I saw the right size was up for auction. Soon the jacket was mine, for a quarter of its original shop price. Did the fact that I couldn’t have it originally make it more desirable? What is the force of delayed gratification? And here’s another thing: I unstitched a black army-style badge that was on the left sleeve, because it looked too like something the SS might have worn and it made me uncomfortable. Don’t let anyone tell you that what we wear is a simple matter.

How ideas spread is a subject that fascinates the modern world, not least because if you understand and control that process, you can wield great commercial or political power. Memetics, which dates from the 1980s, uses the model of evolution to explain how cultural information gets transferred. It comes from the word “meme”, coined in 1976 by Richard Dawkins in “The Selfish Gene” to describe a unit of cultural inheritance that carries ideas, practices or behaviours – like a gene, though without any physical existence. Like a gene, it competes, gets passed on and mutates. Just as, in fact, the word meme has done.

Malcolm Gladwell, the writer of the (itself very influential) book “The Tipping Point”, subscribes to the epidemiological model of influence, writing that “ideas and products and messages and behaviours spread just like viruses do”. He uses fashion as one example. In the early 1990s, a handful of hipsters in downtown Manhattan started wearing Hush Puppy shoes. Within a couple of years a trend that started on a few New York feet had spread to every shopping mall in the country: America was infected with the Hush Puppy virus.

Nicholas Cristakis, a professor at Harvard Medical School, develops the virus model still further: products, behaviours and sneezes are all spread by “networked contagion”, and the closer you are to the centre of the network, the more contact you have and the more infected — or affected — you will be. And because networks are no longer physical but virtual things, because we don’t catch ideas as we once caught cholera from living near an infected water pump, geography becomes almost irrelevant: you are not necessarily more likely to succumb to a trend if you live on the Champs Elysées than if you live in the Outer Hebrides.

The disease analogy may work on paper, but when it comes to fashion its emphasis doesn’t feel entirely right. It can’t depict intent, and there are armies of people whose sole objective is to exert influence. Also it renders the consumer entirely passive. A virus chooses us, whereas when we go shopping we feel we’re doing the choosing. Don’t tell the advertisers, but we do have some free will left.

Perhaps the whole thing works more like the spread of a religious faith, but speeded up. There is the new idea and its disciples, the so-called fashion-forward, like those hipsters in New York. There are the missionaries, the stylists and writers of the fashion media, who spread the word. There are the converts, the consumers who follow, some enthusiastically, others tagging along because they don’t want to be left behind. And there are the atheists, people who are just not interested, who do not have a garment-shaped hole in their lives. Though even they are unlikely to escape entirely unscathed. As a colleague laments: “I don’t read fashion magazines, but somehow it filters through, and suddenly one day I feel that I can’t live without a biker jacket.”

In 1962 — before memes were even dreamt of — the sociologist Everett Rogers wrote that “people follow the lead of other people they know and trust when they decide whether to take [something] up.” When we asked some of our readers, for our Fashion Manifesto last year, where their ideas about how to dress came from, the influence that most were prepared to acknowledge was exactly this: friends, colleagues, people they see as they go about their daily lives. You have only to look at a group of teenagers to see how tribal dressing still is: it’s about belonging and being accepted. Our so-called style icons — the Twiggys, Hepburns and Mosses of fashion history — are not so much influences as lenses: through the simple accident of physiognomy, they focus a group’s vague feelings about what looks right, right now.

We talk about the dictates of fashion as if they came down from on high. Read the interviews on these pages, though, and you’ll see how many of the fashion industry’s officer class are looking down at the rank and file for their ideas (which in their turn, as we have seen with my military jacket, have complex origins). Fashion comes from above, but only in the way that rain falls from the sky: the water that falls down has first evaporated up. From the street to the fashion gods up in the clouds, then back down to the street again: influence is a giant feedback loop. That thought should reassure us about our own role and our creativity. So next time you feel like a fashion victim remember this: you’re part of what makes it happen. The goose that lays the golden meme is waddling around in your own backyard.