Fashion is a problem for evolutionary psychologists, who have to try and explain it. As Matt Ridley points out in his excellent book The Red Queen, the fact that women are so interested in wearing the right shoes and this season's colours is a bit of a mystery. The problem is this: evolutionary psychology insists that if humans have some innate shared instinct, then this must have evolved, and so must have some bearing on breeding success. It is undoubtedly the case that British women are far more interested in fashion than the men here. Specifically, they are interested in women's fashion. This would be easy to explain if men were interested in women's fashion, but they are not. If men were interested in women's fashion, then women could signal their quality to men by wearing the latest togs, and men would select mates based on these signals. The snag is, that men don't. Men will near enough always prefer a beautiful healthy young woman in last century's fashions to an ugly diseased old woman in top modern designer wear.
So if men do not select women according to their fashion sense, why do women bother? Fashion has a cost, and that cost has to be borne by some advantage. If women do truly have an innate fashion interest, and men truly have none, then the war must be, I suggest, between women. Women are not trying to attract men by wearing fashionable clothes, but they are trying to give themselves an edge over potential rivals.
Go into a shop selling magazines today in Britain, and you'll see hordes of magazines selling fashion tips to women, and a handful selling them to men, and the men's magazines haven't been going nearly so long, nor sell so many. There is also the phenomenon of the "lads' mag" in which busty young things are compared for their fleshy pneumatic qualities, and nowhere does it tell you where you can buy the bikinis they are wearing. These magazines are targeted very definitely at their audiences, and men are positively repelled by the women's magazines, just as women are put off by the men's. The greatest likelihood is that these magazine producers know what they are doing, and are going with the grain of human instinct, rather than against it.
In a monogamous world, men will find young women especially attractive. My reasoning is this: if men have to commit to one woman for life, and not have sex before marriage, then the men who breed most will be the ones who pick young wives. Conversely, in a relaxed free-loving polygynous society, the men who do best will be the ones who have broader tastes, as these will have the most opportunities for fathering children.
Let us start, though, with monogamy. In monogamous societies, I would expect there to be massive emphasis on youth when it comes to judging women's beauty, and I would expect women to be more interested in fashion than men. Men would pick women according to their fertility, and unless women came up with a way to mimic youth and health that could fool men, a woman's fashion sense would be of very little help to her in making herself attractive to men.
However, a woman's fashion sense would not be useless. Imagine a society in which there are not very many women around who are available. Such a society was the one in which we evolved. In the world of the forager, a potential mate came along seldom, and one usually only had a small selection of women from which to choose a life partner. In this society, a woman who had known the local men for ages and was on good terms with them, was very nice, but perhaps not the best looking or a little bit past her prime, might lose out to some slip of a thing who walked out of the forest. A foreign woman might out-compete all the local women, even if she hardly spoke the language, if she was young and pretty, and this would not please the local women. What could these women do to keep the stranger out? Well, just as language seems to have evolved partly to keep out outsiders (a human who has learned a language as an adult will almost never fully master it and pass as a local), so too could fashion. The local women could make it next to impossible for the foreigner to pass as a local, and become accepted in society, by coming up with many arbitrary and subtle rules of fashion.
Does this fit the facts? Well, no one really knows how many affairs men of the past had, but it seems reasonable to imagine that in the days of the landed gentry, such as in pre-revolution France, the dandyish men who took great trouble to dress in the latest fashions were having more affairs outside of their marriages thanks to this fashion interest. Even though their world was officially monogamous, it was in effect, so far as genes were concerned, polygynous. In some polygynous tribal societies today, the men parade in gaudy face-paints and costumes, and are picked out by the women. Only the men who understand the language of the face painting and costume making get it right, thus excluding outsiders and cutting down competition.
So, that is my theory: that the fact that modern British women are more interested in fashion than men is down to a person's ability to narrow the field of competition by excluding outsiders from the in-group of rivals. In my childhood, there were very few fashion magazines for men, and these generally dealt with the very up-market end of fashion, since only rich men were getting more mates through fashion. Today, marriage seems to be a fair bit less monogamous, and pre-marital sex is so common, that men have started to show some, but not much, interest in fashion.
Consider now the problem faced by aristocratic women of the 18th century. Like the foragers, theirs was a world of few people in the in-group. Their brothers would have little trouble finding wives, because rich aristocratic powerful men, who stood to inherit lots of resources, could always offer that most attractive of things to their potential spouses: security for the children. However, an ugly aristocratic woman might lose out to a pretty chorus girl, because the local master of the manor might decide to hang convention and marry someone he finds nice and pretty, rather than choose from an exclusive pool of nobles. To combat this, women would conspire to make it next to impossible for pretty lower class girls to break into society. They would come up with a thousand rules of etiquette to do with letter-writing, cutlery use, choice of words, poise, dance, and corset wearing. On top of this they could add fashion: requiring a woman to have four new expensive dresses a year, that no chorus girl could hope to afford.
Did this work? Well, yes, but not perfectly. Ugly aristocratic women did often find aristocratic husbands, but not always. Not far from Newcastle upon Tyne is the Bowes Museum, housed in the country mansion of a stupendously wealthy aristocrat who married a French chorus girl. The local female gentry must have been livid. This doesn't disprove my theory, because I never said that any interest in fashion would exclude all outsiders. I only meant that by putting a fair bit of effort into fashion, many people could improve their breeding prospects by making it more difficult for outsiders to muscle in, and this is what I observe in the world.