A general guideline these days is that people are rewarded when they can do things that take trained judgment and skill — things, in other words, that can’t be done by computers or lower-wage workers in other countries. Money now flows around the world so quickly, and technology changes so fast, that people who thought they were in high demand find themselves uprooted. Many newspaper reporters have learned that their work was subsidized, in part, by classified ads and now can’t survive the rise of Craigslist; computer programmers have found out that some smart young guys in India will do their jobs for much less. Meanwhile, China lends so much money to the United States that mortgage brokers and bond traders can become richer than they ever imagined for a few years and then, just as quickly, become broke and unemployed.

One of the greatest changes is that a college degree is no longer the guarantor of a middle-class existence. Until the early 1970s, less than 11 percent of the adult population graduated from college, and most of them could get a decent job. Today nearly a third have college degrees, and a higher percentage of them graduated from nonelite schools. A bachelor’s degree on its own no longer conveys intelligence and capability. To get a good job, you have to have some special skill — charm, by the way, counts — that employers value. But there’s also a pretty good chance that by some point in the next few years, your boss will find that some new technology or some worker overseas can replace you.

Though it’s no guarantee, a B.A. or some kind of technical training is at least a prerequisite for a decent salary. It’s hard to see any great future for high-school dropouts or high-school graduates with no technical skills. They most often get jobs that require little judgment and minimal training, like stocking shelves, cooking burgers and cleaning offices. Employers generally see these unskilled workers as commodities — one is as good as any other — and thus each worker has very little bargaining power, especially now that unions are weaker. There are about 40 million of these low-skilled people in our work force. They’re vying for jobs that are likely to earn near the minimum wage with few or no benefits, and they have a high chance of being laid off many times in a career.

Global trade and technology are significant trends, but they’re not laws and policies. The actual rules have also changed notably since the 1970s. Back then, there were all sorts of stabilizers that pushed working-class wages up and kept rich people’s wages lower. The minimum wage, at its pre-1970s peak, was almost 50 percent higher than it is now (inflation adjusted, naturally). Unions were stronger and had more government support. The United States taxed the rich much higher relative to the working class. (The top bracket was taxed at 70 percent in 1978; now it’s 35 percent.) It’s hard to imagine, but regulations largely limited the profitability of banks and kept bankers’ financial compensation low.