The rise in the number of Americans not working because of disability was so persistent for two decades that some economists began to hypothesize that the trend would never reverse.

But perhaps it has. Since a peak almost four years ago, that number has steadily fallen, showing its largest decline — both in terms of head count and percentage — in at least the last 25 years. It’s good news, but it also raises important questions about how much further the labor market has to heal.

Disability and a shrinking work force

The employed share of the population 25 to 54 years old — the age range economists generally consider a person’s prime working years — is still almost a full percentage point below where it was on the eve of the Great Recession, and more than two percentage points below where it was before the 2001 recession.

One factor was a steady increase in the number of people not participating in the labor force because of health problems or a disability. In 1994, the Bureau of Labor Statistics determined that 4 percent of Americans 25 to 54 were not seeking work because of those reasons. By mid-2014, the number had risen to almost 6 percent of that age group.