On average, women 65 years and older rely on a median income of around $16,000 a year -- roughly $11,000 less than men of the same age, according to a Congressional analysis of Census data. And many elderly women rely exclusively on Social Security benefits.
The problem: Women earn -- and save -- less over their lifetimes than men, leaving them with a smaller nest egg. And because they tend to live longer, that savings has to last longer, too.
"You combine lower resources with longer life expectancies and very quickly you can identify that there is more risk here," said Dave Littell, retirement income program director at The American College, an institution which educates financial planners and advisers.
Also, female workers make up about two-thirds of all part-time employees. And the majority of those jobs don't come with employer-sponsored retirement benefits.
That makes it harder to save for retirement at all, let alone to accumulate a nest egg large enough to last decades.
Proposals to help workers who do not receive workplace retirement benefits, such as Obama's new myRA plan, are a start but unlikely to make any major improvements.
Gaps in employment: On average, women work 12 years less than men do over the course of their careers, according to the AARP Public Policy Institute.
A major reason: Women are more likely to take time off work to raise kids or to care for a sick spouse or aging parents.
Barbara Bennett, 65, left a high-paying executive position a decade ago to help care for her sick husband, who needed full-time care. But since her husband passed away six years ago, she has struggled to find work.
"What do I do on the top of my resume: Full-time caregiver to deceased spouse?," she said.
And since Social Security benefits are based on a worker's top 35 years of earnings, time out of the workforce can result in a smaller benefit throughout retirement.
One possible fix: to provide "caregiver credits" towards Social Security for workers who are providing unpaid care, said Barbara Butrica, a senior fellow at the Urban Institute. But while the proposal has been made in past Congressional sessions, it hasn't gained much traction.
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An outdated Social Security system: The Social Security program was designed to support married couples where one spouse was the breadwinner. Yet today, a growing number of women are getting married later in life, remaining single or have gotten divorced, and many more women are earning their own benefits in the workforce.
As a result, far fewer women receive spousal benefits based on their husband's work history, according to a Government Accountability Office report. That's good news for women who have earned a lot over the years, but bad news forwomen whose lifetime earnings are low.