Ms. Jacobs and Mr. Shelley have made careers of their nontraditional jobs, meaning both have spent the bulk of their later earning years forgoing benefits. According to the Boston center’s study, that lands them among the group of future retirees likely to see the highest rate of lost income: Their peers are apt to be 26 percent richer because they won’t have pulled from their own pockets to pay for health coverage or have interrupted the accrual of money in their 401(k)’s or pensions.

Making a career of these jobs also puts them among the majority of workers in those positions, who stay put rather than using them as stopgaps between jobs with benefits or bridges to retirement. Workers who bounced among mostly traditional jobs, with stints at nontraditional jobs in between, had about 6 percent less retirement income, the study found.

“Some people do use these jobs to tide them over between one traditional job and another,” Dr. Munnell said; those workers make up only about 25 percent. Fifty-four percent of the 4,174 respondents in the center’s study stay in their no-benefits jobs for years. Among them are people whose lack of a high school diploma prevents them from securing traditional jobs. Others are in Ms. Jacobs’s position, with a spouse to fall back on.

“For those two groups, there’s some logic in working nontraditional jobs,” Dr. Munnell said. “But a third group — they’re as well educated as the average worker, but they’re solo earners. We don’t understand why they’re there.”

Regardless, they will likely feel the pinch later. “It’s important in the later years of your life to be participating in a retirement program, because typically those are the years when accrual is most significant,” said Dan Doonan, executive director of the National Institute on Retirement Security. “It’s really your last chance to build up a nest egg.”

A January report from the institute found that 40 percent of Americans over 60 draw income from Social Security alone, while only 7 percent pay their bills through a combination of Social Security, a defined-benefit pension and a contribution plan such as a 401(k).

That may be because only about half of U.S. workplaces offer their employees retirement plans, Mr. Doonan said. “Access has been a challenge,” he said.