By last year, family income was 8 percent lower than it had been 11 years earlier, at its peak in 2000, according to inflation-adjusted numbers from the Census Bureau. On average in 11-year periods in the decades just after World War II, inflation-adjusted median income rose by almost 30 percent.

Matching the growth rates of the postwar period — when the country was poorer, when harsh discrimination against women and minorities was receding and when the rest of the world was weaker — is probably impossible. Yet there is still a vast difference, both economically and politically, between incomes that are rising modestly and not at all.

Historically, periods of economic stagnation have tended to bring pessimism, political turmoil and a lack of social progress, said Benjamin Friedman, an economic historian and the author of “The Moral Consequences of Economic Growth.” The political volatility and partisan rancor of the last several years seem to fit the pattern.

The recent stagnation has also led, economists say, to confusion and even scapegoating about the real sources of the problem. The causes that can seem obvious, and that often shape the political debate, are not necessarily the correct ones.

Take immigration, especially illegal immigration. Whatever other problems it may cause, evidence suggests that it has not played a significant role in the income slump.

It may have caused a slight decline in the wages of native-born workers without a high school diploma (and maybe not even that). But most illegal immigrants lack the skills to compete with the bulk of native workers, according to research by Giovanni Peri, Chad Sparber and others. Notably, incomes in some states with large immigrant populations, like California, have risen faster than in states with relatively few immigrants, like Ohio.

The minimum wage, similarly, appears to play only a minor role in the income slump. It has risen faster than inflation since 2000, even as overall pay at the bottom of the income distribution has not. And the size of the federal government also looks like a dog that is not barking: Washington collected taxes equal to 15.4 percent of gross domestic product last year, down from 20.6 percent in 2000.