...To fix it, executives came up with what, for Walmart, counted as a revolutionary idea. This is, after all, a company famous for squeezing pennies so successfully that labor groups accuse it of depressing wages across the American economy. As an efficient, multinational selling machine, the company had a reputation for treating employee pay as a cost to be minimized.

But in early 2015, Walmart announced it would actually pay its workers more.

That set in motion the biggest test imaginable of a basic argument that has consumed ivory-tower economists, union-hall organizers and corporate executives for years on end: What if paying workers more, training them better and offering better opportunities for advancement can actually make a company more profitable, rather than less?...