It's an attention-grabbing statistic. And one expert in particular, Andrew Biggs, resident scholar at the American Enterprise Institute, finds it hard to believe.
Much of the effort to reform Social Security comes down to numbers. Because the stakes for reform are so high, it's no surprise that researchers are also at odds.
The National Institute on Retirement Security's report specifically looked at respondents or households ages 60 and up who work fewer than 30 hours per week or not at all.
The research is based on 2014 data from the Survey of Income and Program Participation and the Social Security Administration Supplement on Retirement, Pensions and Related Content.
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But Biggs at the American Enterprise Institute doubts that that many people really only have income from just Social Security.
Their numbers, according to Biggs, are actually much smaller, based on data from the Census Bureau.
"The share of people who only have Social Security is a lot smaller than you think it is," Biggs said."It's not 40%. It's 12%."
Among Biggs' complaints is that the National Institute on Retirement Security skews its data by taking individuals who are age 60 and over, which can include individuals on disability benefits. It also potentially overlooks individuals who are delaying retirement, he said.
Research from other sources, such as from the Census or the Social Security Administration, typically looks at individuals who are 65 and older.
The results are one example of how research can paint an unrealistically negative picture for retirement savings, according to Biggs.
"Over several decades, retiree incomes have been growing much faster than incomes of working age households," he said.
Still, the National Institute on Retirement Security, otherwise known as NIRS, stands by its research.
The researchers didn't use a Census Bureau study that was published in 2017 because there were concerns about the reliability of the data, said Tyler Bond, research manager at NIRS.
NIRS set out to look exclusively at sources of retirement income. To do that, the group purposely excluded people who might still be working full-time, who would receive much of their income from wages. It also includes people who might not yet have reached eligibility for Social Security retirement benefits, which generally starts at age 62.
That criteria led to different results from other studies, according to Bond.
"We feel confident in our numbers," Bond said. Moreover, data from the Social Security Administration can also help back up their findings, he said.
NIRS' research was peer-reviewed by fellow researchers, including Christian E. Weller, professor of public policy at the University of Massachusetts Boston. Weller said he stands by the research.
"Andrew's overarching argument, that he can probably make better than I can, is that there is nothing to worry about, that we can tinker with the current system, and we'll be fine," Weller said, referring to Biggs' objections. "I don't think anybody who realistically studies retirement savings believes that."
There is growth of economic insecurity among older households, and their savings relative to income is much less over time. That points to a foreseeable problem, Weller said.
"There is a growing share of people, regardless of the data set and regardless of the methodology used, who are ill prepared for retirement," Weller said.
The conflict comes at a time when Social Security reform is being debated. And any changes that Congress makes to the program will likely be based on available research.
That's what makes Biggs, who previously worked for the Social Security Administration during former President George W. Bush's administration, so adamant about correcting data he sees as erroneous. These statistics tend to get repeated at congressional hearings, he said.
Biggs, for his part, is for expanding benefits for people on the low-income end.
"We should have a true guaranteed minimum benefit that keeps everyone out of poverty," Biggs said. "It's not that expensive to pay for."
One congressional proposal, called the Social Security 2100 Act, aims to reform the program by increasing taxes, while providing across-the-board benefit increases. That bill has more than 200 Democratic co-sponsors in the House. Meanwhile, Democratic presidential candidates have put forth their own proposals on how they would revamp the program.
While Biggs maintains that many retirees are actually richer than studies indicate, NIRS maintains that there is still a wealth gap that needs to be addressed.
"The distribution of retirement income matters," Bond said.
While retirement income and wealth have increased in recent decades, so has income and wealth inequality, Bond said.
"If the majority of retirement wealth is concentrated at the top and not evenly distributed, then just saying average retirement wealth has increased doesn't really tell the full picture," Bond said. "That's part of what we were trying to tell with this report."