After decades of growth, union membership in the United States has been declining steadily for the past 20 years, according to the Bureau of Labor Statistics. Total membership peaked at almost 18 million people in 1980. By last year, according to The Charlotte News & Observer, that figure had dropped to less than 16 million, just 13 percent of the country’s workforce.

It’s happening everywhere. In Minnesota, the most heavily unionized state in the Upper Midwest, membership declined by 17.6 percent between 2002 and 2003, according to the St. Paul Pioneer Press. North Carolina, the least unionized state, saw its membership decrease by 26 percent from 1995 to 2004. Only 2.7 percent of employees in that state were union members last year.

While Wisconsin’s union membership increased a few tenths of a percentage point last year, the national trend is clear: A mere 12.9 percent of U.S. workers were union members last year, down from 13.3 percent in 2002. Even in the most unionized states, such as New York, Michigan, Alaska, and Hawaii, union members account for only 22 to 25 percent of the work force.

According to the Baltimore Sun, this has triggered a debate about what started this downward trend in union membership. The slide coincided with Ronald Reagan’s presidency, but scholars and economists, as well as union organizers themselves, are unsure if Reagan’s policies were responsible for the shift, or if they simply accelerated a trend that was already underway.

The fact is that union membership peaked in the 1950s at about 35 percent. By the time Reagan became President in 1980, that figure had already dropped to 23 percent. It’s true that the absolute numbers of union members grew up to that point, but they weren’t keeping pace with the expansion of the workforce.

It appears that, if anything, Reagan gave a slight push to an already developing cultural and societal trend against unions when he fired 12,000 air traffic controllers in 1981. This sent the message that it was all right for business to stand firm against unions. But the unions were already in trouble by then, and it was largely trouble of their own making.

A bit of history is in order.

The roots of our country’s trade unions extend deep into the early history of America. Several of the Pilgrims...