Workers aren’t planning on early retirement.

In fact, one out of every three plans to stick it out in theworkplace past the age of 65, likely because they aren’t confidentthat they’ve saved enough money for retirement.

But that later retirement’s not happening, as many are compelledto retire earlier than they’d anticipated because of shocks thatthrow off their plans.

Read: 10 questions to ask about retirementpreparedness

According to research from the Center for Retirement Research atBoston College, health issues are the chief cause driving workersinto retirement earlier than they’d planned on going.

Read: Women have to work harder in retirementsaving

After that came layoffs—it’s tough to stay in the workplace ifyou don’t have a job—and the third most common reason for earlyretirement was a spouse’s early retirement or having a parent movein.

Between 1991 and 2014, the percentage of workers indicating thatthey planned on working past age 65 increased from 11 to 33percent.

However, despite knowing that early retirement will result insmaller Social Security checks and fewer years to beef up 401(k)s,while at the same time increasing the number of retirement yearsfor which expenses must be paid, workers are leaving the workplaceahead of schedule.

The study sought to learn why.

The research analyzed the responses of workers who wereage 58 at some time between 1992–2012and were asked when they expected to retire.

Researchers then followed each person for up to a decade todetermine who retired, whether the retirement was early or onschedule, and what intervening life changes might have caused achange in their stated plan.

The changes tracked in each person’s health and cognitiveabilities included stroke, diabetes, cancer, and difficultydressing and reading a map.

The most important factor that emerged was health, in twodifferent ways.

First, workers in bad health when making their retirement plansare more likely to retire earlier than others, even if their healthdoes not change.

And second, a deterioration in health leads toearlier-than-expected retirement.

While job loss came in as the second reason people retiredearly, researchers determined that moving from one job to anotherimproved the likelihood that workers would be able to follow theiroriginal schedule for retirement—but only if the new job was animprovement over the old: better pay, fewer hours, or lessstress.

If the new job was worse, researchers found, workers wereactually much more likely to retire early.