In Denmark, the share of income going to the top 1 percent rose to 6 percent from just 5 percent. In the Netherlands, there was essentially no increase from 6 percent levels. Britain (6 percent to 14 percent) and Canada (9 percent to 14 percent) had notable increases in top-income earnings, but not as large as those in the United States .

Before looking into some of what’s behind this, let’s address common misconceptions.

No, It’s Not Trade

A rise in international trade — as a share of G.D.P., measured as either imports or exports using data from the Penn World Tables — is associated with equality, not inequality. The United States imports only a small fraction of the value of its total economy, whereas Denmark and the Netherlands are highly dependent on imports.

Or the Rise of Information Technology

Countries with higher rates of invention — as measured by patent applications filed under the Patent Cooperation Treaty, an indicator of patent quality — exhibit lower inequality than those with less inventive activity. As it happens, tech industries in the United States have contributed just a tiny bit to the rise of the 1 percent, and the salaries of engineers and software developers rarely reach the 1 percent threshold of an annual income of $390,000.

What About Unions?

Unions are thought to redistribute income from owners to workers, but there is no correlation across countries between the change in labor’s share of G.D.P. since 1980 and an increase in the income share of the top 1 percent. Britain saw an increase in the labor share of G.D.P. but also one of the sharpest increases in inequality. The Netherlands saw a large fall in labor’s share but no rise in inequality.

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Scandinavian countries are heavily unionized and egalitarian, but Denmark experienced a large decrease in the share of workers represented by unions from 1980 to 2015, according to O.E.C.D. data, and very little change in inequality. Unionization rates dropped precipitously in the Netherlands and especially New Zealand over the period, but inequality rose as much if not more in Spain, where unionization rates rose.