Economist Allison Schrager points out a little-noticed shift in the workforce that explains part of the membership problem Big Labor is facing: Union members are now experiencing declining job security.

This is notable because job security is a key benefit to joining a union in the first place. Unions are supposed to pressure companies to preserve positions, even in hard times. But even union-dominated industries have seen mass layoffs in recent decades.

Also, many of the other benefits that unions provide, like pensions, are only available to people who remain at their jobs for set periods. Without the assurance that they will be at a particular job for a long time, the other benefits unions provide to workers are less of enticement to become a member.

In a piece on Quartz, Schrager compared the median age and tenure of workers in 1987 and 2007. The results: In 1987 the average union member had a median tenure of 10 years on the job and was 41 years old. By 2007, the median tenure was still 10 years but the median age had risen to 46.

It was even worse for union members in manufacturing. Their median job tenure fell from 13 to 10 years while the median age rose from 41 to 50.

She noted: "Aging combined with falling (or constant, for the entire union population) tenure shows union members’ employment relationships have gotten shorter. It’s worth noting that union members still have longer tenure in their jobs compared to people with no affiliation, but some of that can be explained by age."

The workforce has largely adapted to this change. Most workers, especially younger ones, no longer expect to work at the same job for decades. Many view this as an advantage, not wanting to be tied to the same job indefinitely. Federal labor law, by contrast, remains structured around people staying in the same workplace for their entire career.

Schrager's analysis used data from the Federal Reserve's Survey of Consumer Finances. She eschewed more recent data, arguing it would have reflected the recession, not long-term trends.