I've been thinking where else to apply the back-to-nature principle, which goes like this: if we're disturbing the outcome of natural selection, this practice is guilty until proven innocent. In some cases, a good case can be made -- like washing your hands. True we didn't evolve in a world with soap, and we are thus messing around with our natural state. However, we live in a world that's far more germ-ridden than when we all lived on the African savanna, mostly due to crowding, contact with animals, and living -- and doing our business -- in the same place over time. So, severely cutting down on the concentration of harmful junk on our hands gives a huge boost to our health.
That does not apply to shampoo, though. It has no anti-bacterial, anti-fungal, or anti-viral powers, unlike soap. (There are special shampoos if you want to zap the bacteria that contribute to dandruff or if you have lice, but I'm talking normal everyday shampoo.) All it does is remove some of the dirt and dust that sticks to the oil that your scalp makes and that coats your hair follicles (sebum), so there's no sanitation or hygiene angle here. Yet it doesn't have laser-like precision in removing these dust particles -- it strips away a good deal of the sebum too.
Now, natural selection put oil-making sebaceous glands on your scalp for a good reason -- and you don't even have to know what it is, although it's fun to speculate. It survived natural selection, so it must have been doing something good for you. And people can easily tell that their hair gets messed up in some way (too dry, too limp, whatever) after shampooing, so they then try to put some of the moisture back in with conditioner. Whenever you need a corrector for the corrector, you know you're digging yourself deep.
Washing the dirt and dust out of your hair doesn't take anything more than a thorough rinse in a shower with good water pressure. This is what humans have done for as long as they've had access to bodies of water in which to bathe, and the hair of hunter-gatherers looks just fine. They've also infused oily substances into their hair -- olive oil in the Mediterranean, coconut oil in the South Seas, a butterfat mixture among the Himba pastoralists of Namibia, and god knows what else. But whereas rubbing fatty stuff into the hair appears universal, using cleansing agents just to get out a little dust hardly shows up at all.
That's true even in the industrialized West. I searched the NYT archives for articles mentioning how often you should shampoo. There aren't any such articles before the first decade of the 1900s, when the consensus was that once a month was good, but aim for once every 2 to 3 weeks. That was true for the 1910s, too. I couldn't access the articles from most of the following decades, although there is one from the WWII days that mentions women's weekly shampoo. Brief histories I've read suggest that the advertizing of the '70s lead to even more frequent shampooing, and certainly I remember from personal experience that by no later than the end of the '80s, daily shampooing was expected.
As much as I rave about that decade, no one did babes like the '60s. Everything lined up just right for them -- lots of animal products in the diet, little / infrequent shampooing, no layers of hairstyling products, normal amount of sun exposure, and little or no air conditioning = normal level of sweat, helped out by a normal mix of physical activity. God, that big bouncy hair... here is a cropped, safe picture of Cynthia Myers, Playmate of the month for December 1968. You won't see that in the era of Clueless and Mean Girls. In fact, if it weren't for styling products, I doubt you would've seen it in the '80s either.
As for guys, here's a picture comparing current star Zac Efron to Leonard Whiting, who played Romeo in Zeffirelli's 1968 movie. Whiting's hair looks thicker at the individual hair level, more voluminous overall, and more lustrous.
It's been almost two weeks since I last shampooed my hair, although I still rinse it well with just water in the shower, and it's not a big greasy mess like I imagined it would. After a lifetime of daily shampooing, it'll take my sebaceous glands some time to adapt to the *lack* of the oil-sapping stuff and dial down their activity. From what I've read, it'll take between two weeks and two months for a complete return to normalcy, but it's worth it. Reading around, I was struck by how wimpy people are when they try this out -- "omigod i could never go for more than like three days, i'd feel so gross!" They know that it will all work out, but we've become so focused on immediate comfort rather than enduring robustness. It's considered a violation of someone's human rights to tell them to deal with it until it gets better.
There seems to be an eco-friendly movement afoot called "no 'poo" -- those damned Greens will never learn good advertizing -- which aims to reduce shampoo use for some environmental reason or other. It seems like most of them still use a cleansing agent and conditioner to ameliorate the damage done by the cleanser, though. Knowing that they're eco-friendly, we can infer that they ingest little animal fat and protein, on which our hair is so dependent, so they're not the best example of what little or no shampoo looks like -- for that, have another look at those '60s honey bunnies.