In the Washington Post today, MIT professor Simon Johnson and Yale law student James Kwak argue that Americans relying on Social Security and a 401(k) plan won’t have nearly the income required to enjoy a decent retirement. Their numbers are wrong and their conclusions erroneous. But it’s not their fault: their problem was relying on the Social Security Administration’s (SSA) online benefit calculator, which significantly understates the true value of future retirement benefits. This error in the SSA benefit calculator is, to be frank, something the agency is aware of yet has failed to correct.
SSA’s online calculators—and until recently, the benefit statement sent to more than 100 million Americans—says that benefit are expressed “in today’s dollars,” meaning adjusted for inflation. But, as I wrote in the Christian Science Monitor last year, benefit estimates produced by SSA are not in today’s dollars. Rather, benefits are expressed in “wage indexed” dollars, which significantly understate the true inflation-adjusted value of benefits. Not knowing that, Johnson and Kwak, and countless Americans, underestimate future retirement benefits.
Johnson and Kwak simulate the Social Security benefits and 401(k) income for a couple with typical earnings patterns and 401(k) contributions. Their Social Security benefits and an annuity purchased with their 401(k) balance at age 65 are compared to their earnings at age 64 to calculate a “replacement rate.” Financial advisors recommend replacing 70 percent to 80 percent of pre-retirement income to have a comfortable standard of living.
Johnson and Kwak state that, “according to a Social Security benefits calculator, each adult would earn a $1,122 monthly Social Security benefit on retirement, for a total of $26,928 per year. (All figures are in 2008 dollars.)” This produces a replacement rate of 35 percent of earnings at age 64.
Johnson and Kwak next assume that the couple contributes 2.4 percent of their earnings to their 401(k) and earns a return of 6.4 percent above inflation each year. This produces a final 401(k) balance of $298,064, sufficient to pay $18,467 per year for life. In total, Johnson and Kwak calculate a replacement rate of just 59 percent of pre-retirement earnings, far short of the recommended 70 to 80 percent. Just more evidence that Americans are ill-prepared for retirement, right? Actually, wrong.
Johnson and Kwak’s figures struck me as too low, given previous work projecting replacement rates for future retirees, so I replicated their results. I used SSA’s “scaled earners,” which simulate typical earnings patterns for individuals of different wage levels. (SSA’s scaled medium earner has slightly above average earnings and so will produce slightly higher benefits than Johnson and Kwak’s numbers, but the differences are small.) Using these earnings, I projected Social Security benefits and 401(k) balances. I then compared the Social Security benefits and 401(k) income to earnings at age 64 to calculate replacement rates.
Here’s what I found on Social Security replacement rates: a low earner would have a replacement rate of 70 percent, a medium earner 52 percent, and a high earner 43 percent. All would receive replacement rates of 24 percent from the 401(k). So total replacement rates are 94 percent, 76 percent, and 67 percent, respectively.

I found a total replacement rate for a medium wage earner of around 76 percent while Johnson and Kwak found only 59 percent. What drives the difference? It’s not the 401(k) plan, where our numbers are similar, but the Social Security benefits: my replacement rate was 17 percentage points higher than theirs.
Recall that Johnson and Kwak estimated their Social Security benefit of $1,122 in 2008 dollars using SSA’s online benefit calculator. The calculator shows benefits “in today’s dollars,” which Johnson and Kwak reasonably take to mean inflation-adjusted dollars. My estimated benefit in today’s dollars, however, was $2,235, almost twice as much. Here’s what happened: the medium wage earner I simulated would receive a nominal benefit of $7,034 per month in 2051; “nominal” means the actual payment in dollars on the benefit check. Using the CPI deflators from the annual Trustees Report, $7,034 in 2051 would be worth $2,235 in 2008 dollars, far more than what Johnson and Kwak report. However, if we wage-index the $7,034 benefit back to 2008—which means multiplying the nominal benefit by the ratio of the average wage today to the average wage in 2051—it comes out to $1,411, similar to Johnson and Kwak’s figure (the difference is due to our slightly different earnings assumptions). The wage-indexed benefit amount is equal to just 63 percent of the inflation-adjusted value.
This difference drives Johnson and Kwak’s entire results. Using benefits that are correctly adjusted for inflation, their replacement rates—and their conclusions regarding American’s readiness for retirement—would be far different. (For what it’s worth, in this paper Glenn Springstead and I calculated replacement rates for retirees in 2020 and find the median value of all retirement income—Social Security, earnings, DC and DB pensions, etc.—would be around 199 percent of final earnings.)
The SSA has been aware of the wage-indexing issue at least since the time of my Christian Science Monitor piece. The agency argues that using wage-indexed dollars makes benefits more comparable to today’s standard of living. (Technically, the ratio of the wage-indexed benefit amount to today’s economy-wide average wage would be the same as the ratio of the future nominal benefit amount to the future average wage. Why people estimating their own retirement benefits would be so focused on average wages today or in 2051 is anybody’s guess.) But anyone planning their retirement saving will project their future income either in nominal dollars or inflation-adjusted dollars. No one—I promise you, no one—will estimate them in wage-indexed dollars. For that reason, benefit estimates from the Social Security Statement and SSA’s online benefit calculators simply can’t be mixed with estimates of 401(k) or other non-Social Security retirement income—they’re simply apples and oranges. When you do mix them you get what Johnson and Kwak got—a significant error that leads to wrong conclusions.
SSA’s action to date on this issue has been to remove the phrase “in today’s dollars” from annual Social Security statements. This at least removes the truly egregious error, even if it remains in the online benefit calculator. But that still leaves Americans to guess in what terms the statement’s dollar amounts are expressed—and no one is going to guess wage-indexed. The fact that an MIT business professor and a Yale law student made this error should tell us everything we need to know.
None of this is to say that Americans shouldn’t save more, particularly since the underfunded Social Security program is likely to reduce benefits in the future. But as I wrote in the Christian Science Monitor, “Higher retirement benefits hidden in the numbers is a nice surprise, but the Social Security Administration should nevertheless correct the statement’s benefit calculations. Right or wrong, Social Security already has credibility problems. For Americans to accept the sacrifices involved with reform, they need confidence that official numbers coming from the agency are accurate.”