Summary: if you google for why humans get overcrowded teeth and need expensive orthodontics in their teens, you get unhelpful advice telling you that tooth overcrowding occurs when the teeth are too large for the jaw. Recent research strongly suggests that chewing tough food results in larger jaw development, and it seems likely that modern problems with tooth overcrowding are due to a modern diet of relatively soft food.

Recommendation: make sure children's diets include tough food, not just food softened with modern processing.

I started researching tooth overcrowding because I was curious about Weston A. Price's theory that the modern Western diet (particularly flour, sugar, and modern processed vegetable fats) causes nutritional deficiencies that are a cause of many dental issues and health problems, including small jaw development leading to tooth overcrowding.

I came across this paper, describing an experiment on rock hyraxes, a kind of mammal chosen because its face shape is similar to human faces in some ways. They fed two groups of hyraxes the same kind of food, but for one group it was hardened with a food dessicator, and for the other it was softened with soaking and microwaving. The hyraxes growing up on soft food had narrower faces, and thinner and shorter mandibles than the hyraxes who ate tough food. Fortunately for the hyraxes, their teeth also adjusted size correspondingly. It seems that human teeth don't adjust to masticatory stress, but human jaws do, so you can get normal-sized teeth in a small jaw, leading to tooth overcrowding and expensive dentistry. The paper describes previous research showing the same effect in squirrel monkeys, baboons, and macaques.

Weston A. Price observed that hunter-gatherer societies didn't have problems with tooth overcrowding, and concluded that the nutrition of the modern Western diet was to blame. He was almost right - it seems likely that the softness, not the nutrition, of the modern Western diet, is the primary cause.

I'm convinced, and I emailed Will about it, and he agrees that it's probably correct: soft food causes smaller jaw development and tooth overcrowding. Make sure children's diets include tough food.