With less guaranteed income when they retire, a larger number of older workers remained in the workforce in 2010, according to the Employee Benefit Research Institute.

On Friday, EBRI released a study that found that despite the recession, 40.2% of workers in 2010 were age 55 and older — the highest level in 35 years. In fact, the percentage of Americans age 55 or older who were in the labor force tumbled from 34.6% in 1975 to 29.4% in 1993, before climbing to 40.2% in 2010.

What is most striking is that for the 55-64 age group, the labor-participation rate increase was due to a flood of women in the work force, since the rate for men was either stagnant or declining. Yet for the next highest age group, 65 and older, the rates for both men and women increased. In addition, those with more education were more likely to stay in the workforce than those with lower levels of education. In 2009, 63.1% of individuals with a graduate or professional degree were in the labor force, compared to 22.4% of those without a high school diploma.

EBRI reports that as full social security benefits get pushed back even more, a greater number of older workers will continue to stay in the workforce. In addition, Medicare premiums continue to climb; non-Medicare-covered health costs are rising; people are living longer; more workers are taking care of aged parents; and the number of individuals that are “very confident” of their ability to maintain their lifestyle in retirement is low.

“Older Americans, particularly those who worked in the private sector, increasingly have considerably less access to guaranteed levels of income such as pensions or health insurance benefits when they retire,” said Craig Copeland, EBRI senior research associate and author of the report, in a statement. “And staying in the work force longer will allow them to either build up, or rebuild, their assets.”

The findings are based on the U.S. Census Bureau’s data on labor-force participation among Americans age 55 and older. Work force participation rates have increased across every race and ethnicity group since the mid-1990s. White Americans and those in the “other” category had higher rates of participation in recent years. Black Americans’ rate was slightly below white Americans, while Hispanic Americans had the lowest labor-force participation rate. In 2009, the participation rates continued to increase for white Americans and those in the “other” category, while declining for both black and Hispanic Americans.