Since the 1960s, organized labor in the United States has been steadily decaying. A half-century ago, 30 percent of American workers were members in a union. By last year, that had shriveled to 11.8 percent. Economists have offered up all sorts of theories for the drop, from the shrinking manufacturing workforce to foreign competition that has made U.S. companies more hostile toward unions.

But a new paper (pdf) from Kris Warner of the Center on Economic and Policy Research suggests that the decline in U.S. labor unions wasn't simply due to inexorable economic forces. Government policies likely played a big role too. And the easiest way to see this, Warner argues, is by comparing unionization rates in the United States to rates in nearby Canada, "the country that is probably more like the U.S. than any other – economically, socially, and politically."

Here's the key graph from the paper:

Between the 1920s and 1960s, both countries saw a similar surge in union membership, thanks to changes in labor law and the growth of sectors ripe for organizing, such as automobile manufacturing. But around 1965, something changed. The two countries diverged. Union membership held steady in Canada, but plummeted in the United States.

So why the split? Globalization, after all, has hit both countries. Canada's manufacturing sector has been similarly squeezed by competition from abroad. And the desire among workers to join a union appears to be roughly similar in both countries, judging by survey data.

Instead, Warner suggests, the biggest reason for the two nation's contrasting fates has to do with labor law. Canada's rules for union organizing are largely overseen by its provinces, and, until recently, Canada's rules made it much easier for workers in the private sector to form a union. A majority of employees in the workplace simply needed to sign cards indicating their desire to join—a process known as "card check."

In the United States, by contrast, there's usually a second step involved—a secret-ballot election is held by the National Labor Relations Board, and usually only after a lengthy period in which employers can campaign against the union. "During the time between the petition and the election," Warner writes, "which is often delayed by employer opposition and can last for months, employers usually run anti-union campaigns – often committing illegal acts of coercion, intimidation, or firing – in an attempt to discourage their employees from voting to unionize." Research suggests (pdf) that U.S. employers have become remarkably adept in fending off unionization drives, often with the help of anti-union consultants.

Warner also points to a second key policy difference. In Canada, workers who have formed a union can seek arbitration to ensure that they actually get a contract. By contrast, in the United States, employers have much more freedom to delay that process. As John Schmitt notes, "even after workers win an election, they only reach a contract in a bit over half the cases."

Card check and contract arbitration may well explain the different unionization rates in Canada and the United States. It's worth noting, however, that Canadian unionization rates have still started declining since 2000, albeit more slowly. Part of that may be due to the changing economic landscape. Ken Georgetti, the head of the Canadian Labour Congress, has blamed the losses on the fact that "large manufacturers in Canada have slashed jobs over the past decade – almost all of them unionized positions – while new-economy jobs in sectors such as technology and professional services are much less likely to be union positions." If true, that suggests that unions will decline even when labor law is quite favorable. There's only so much card check can do.

Then again, Warner notes that another factor is that Canadian labor law has actually been shifting slowly toward the American model. In recent years, several provinces, particularly Ontario, have switched toward mandatory elections instead of card check. Although the elections are typically swifter than those in the United States—giving employers less time to mount an anti-union campaign—the formation of new private-sector unions has slowed in these provinces, as labor expert Sarah Slinn has found.

And there's little sign that the United States, for its part, is becoming more like Canada. Back in 2009, U.S. labor unions tried to push legislation that would legalize the card check process, but it died in the Senate after opposition from Republicans and several Democrats. This year's Democratic platform, meanwhile, remains quite hazy on what to do about labor unions: "As new employment relationships evolve away from the traditional employee-employer model," it says, "we need to make sure our labor laws are modernized and keep pace with changes in our economy."