By most measures, there’s a lot to cheer about in the current economic expansion. The economy has been growing at a healthy percentage since the 1983 tax cuts, a sustained market rally has brought the Dow to new heights, and unemployment has plummeted to labor-shortage levels.

Despite this, Democrats continue to fixate on the one statistic they feel throws a pall over these otherwise good times: the increasing inequality of income and wealth between the rich and the poor, or the “Two Americas,” in presidential candidate John Edward’s evocative if misleading phrase.

The gap is indeed real. It’s documented in the U.S. Census Bureau’s annual report on income, and it has been going on at least since the 1970s, long before the Bush tax cuts, which are current putative culprits being targeted by the Democrats.

Recent statistical corrections by the Census Bureau have narrowed the gap somewhat, but there’s no doubt that the trend has been toward a greater disparity between rich and poor over time.

If Democrats were to look behind the statistics, however, and take stock of the social dynamics that are largely driving this trend, they might reconsider their dissatisfaction. Two of the biggest political drivers are immigration and the entrance of women into the market place. Both are phenomenae typically welcomed by the Democrats, and both are significant facts of contemporary American life. Those two issues probably have at least as great an effect as other considerations, including globalism and the rise in high-paying high-skilled jobs in a high-tech economy.

According to the Center for Immigration Studies, 4.1 million immigrant workers entered America between 2000 and 2005, accounting for 86% of the net increase in the total number of employed persons. Some of these are illegal, and many illegals are missed in the Census statistics. Nonetheless the numbers are impressive.

Reason and experience tell us that many of these workers are taking low-skilled, low paying jobs and will almost certainly be counted in the lower quintiles.

One can argue about whether these immigrant workers depresses the wages of native born Americans or by how much, but there can hardly be any doubt that such a large and sustained influx of low-paying labor will weight the statistics toward lower end of the income scales.

Moreover, this is a trend that will likely continue. The Labor Department estimates that between 2004 and 2014 our economy will add millions of net new jobs at the lower end, with the largest growth in categories such as janitors, waiters, food preparation, and home health care workers — in other words, exactly the kind of jobs immigrants typically fill.

The other phenomenon — the entrance of women into the market place — has been quite successful on the level of pure numbers.

In 1950, about one in three American women were working in the labor force. By 1998, nearly three out of every five women were in the labor force. In total, women now account for about 46% of the entire workforce, and the number is expected to keep on growing.

This has a huge effect on the statistics because the Census is counting households, not individuals. Well-off households are more likely to be intact and contain two workers while those households on the lower end of the scale are headed more frequently by one earner, often a single mother struggling to make ends meet.

The female push into the market place will only drive the gap wider in the future. Today, women already account for 51% of all workers in high-paying occupations.

In general, the more education you have, the more money you make. So if you want to look at what’s going to happen 20 years out, check out colleges and graduate schools, where women now heavily outnumber men, making up 56% of undergraduate students and 59% of graduate students in 2005.

So what do the Democrats want to do: Close the doors to immigration? Kick women out of college, reinstitute the glass ceiling in the work place, and send all those women back home?

Short of that, there is no “solution” for income disparity. The huge new tax hikes some Democrats are proposing on capital — such as raising capital gains — will only depress economic growth and make everyone poorer.

Maybe it’s time for the Democrats to stop worrying and start loving income inequality.

Mr. Rabushka is the David and Joan Traitel Senior Fellow at the Hoover Institution at Stanford University and co-author of “The Flat Tax.”