When the sneaker industry got going around the middle of the 19th century, the shoes were a niche product—footwear for the rich sport of lawn tennis (hence the enduring term “tennis shoes”). By now they’ve so far transcended just athletic use that market-research firm NPD Group forecasts “sport leisure” sneakers—the casual sort, as opposed to performance shoes meant specifically for athletic use—will become the largest footwear category in the US in 2020. The incumbent front runner they’ll overtake is what NPD calls “fashion” footwear, a category including most of everything else: shoes, boots, sandals, and slippers. It will mark a major milestone for sneakers, not least of all because the US is one of the world’s two biggest fashion markets. Sneakers have become integral to the way people dress (Quartz member exclusive). They’re so ubiquitous that it may surprise some to learn casual sneakers weren’t already the top category recorded by NPD. But the shoes had a lot of ground to make up. Sneaker makers also haven’t historically paid as much attention as they could have to women, a large and valuable customer group, and as indicated, their numbers also don’t include performance s...