As Sen. Bernie Sanders has begun to make waves in the Democratic presidential primary field, an old quote by the Vermont senator has been making the rounds on social media.

"A two-income family today has less disposable income than a one-income family had 30 years ago," Sanders said, according to a post on Reddit that links to an undated video of Sanders addressing a California Democratic Party convention. The claim was sufficiently intriguing that three readers independently emailed PolitiFact to ask whether it was accurate.

We found several examples of Sanders saying this, or something close to it. He used the line in a 2007 op-ed in the Huffington Post, in a lengthy and high-profile Senate floor speech in 2010, in the 2011 book version of that speech, and in the California Democratic Party address from that video, which, we determined, occurred in 2011.

When we checked with Sanders’ staff, however, they said he doesn’t use this talking point any more, and indeed, we couldn’t find examples of him using it after 2011. For this reason, we won’t put the claim to the Truth-O-Meter. Still, because the claim was interesting to us, and because it was clearly of interest to some of our readers, we decided to take a closer look at its origins and accuracy.

Is something off here?

We first turned to Census Bureau data (table F-12 here, to be specific) that measures median income for families by the number of earners in the family. The figures are adjusted for inflation.

We found that the median income for a one-earner family in 1983 was $43,144. For a two-earner family in 2013, the median income was $87,997. That’s quite a bit higher -- more than double, in fact, in inflation-adjusted dollars -- but that’s total income.

What about "disposable income"?

Economists define disposable income as income minus taxes. We did our own estimate of how much of a bite taxes took out of incomes in both 1983 and 2013 by looking at inflation-adjusted data from the Commerce Department’s Bureau of Economic Analysis. As it turns out, BEA data shows that disposable income accounted for about 88 percent of personal income in both 1983 and 2013. (This is a total figure, of course; individual results may vary.)

Using this data, we calculated that 88 percent of one-earner family income in 1982 was $37,967, while 88 percent of two-earner family income in 2013 was $77,420. That, too, is more than double -- a far cry from being less, as Sanders said.

So what’s going on?

Disposable vs. discretionary

A big part of the problem is that Sanders was using the word "disposable" when he should have said "discretionary." This is a common mistake -- both words start with "d," after all -- but one that has significant consequences for the claim’s accuracy.

Unlike disposable income, discretionary income doesn’t have a widely accepted definition. Generally, it means disposable income minus expenses that are necessary to maintain a decent standard of living. Housing and food would likely be included, and you could make a good case for clothing, health insurance and a motor vehicle.

But there’s no official list and lots of room for disagreement about how much to allow as a reasonable cost burden for such items. Own a home or rent? Whole Foods or Walmart? New or used car? Low- or high-deductible health insurance? And that doesn’t even take into account geography; certain areas of the United States have much higher or lower costs of living.

Brookings Institution economist Gary Burtless noted that unlike taxes, which must be paid under law, the necessity of everything else is more flexible.

"Nearly everyone needs shelter, but relatively few of us are obliged to spend 40 percent or more of our income to obtain a minimally decent place to live," Burtless said. "Just because people spend a huge fraction of their income to live in a bigger house or a more expensive neighborhood does not mean they face a mandate to pay so much for their housing. There are usually less expensive alternatives."

Tara Sinclair, a George Washington University economist, added that the structure of households has changed over time. "Families are smaller, and two-earner households are more likely among the higher educated who are more likely to get married," she said. "So the numbers are slippery."

The Warren connection

As it turns out, we were able to locate what may be the methodology that produced Sanders’ (former) talking point. Interestingly, the source may have been Sanders’ fellow Senate progressive icon -- Elizabeth Warren, D-Mass. -- and, even stranger, it emerged long before either politician ended up in the Senate.

The comparison appears to have come from a passage in Warren’s 2003 book, The Two-Income Trap, co-authored with her daughter, Amelia Warren Tyagi. It compares two families, one in the 1970s and one around 2002. Each has a mom, a dad, and two kids. In the 1970s, the mom stayed at home; in 2002, she was working, so the 2002 figures included costs for a second car and day care, plus taxes for a higher tax bracket. All figures were inflation-adjusted and were drawn from median figures for each category.

The 1970s family, by the Warren calculation, earned $38,700 in income from the father, and had to pay $5,310 in annual mortgage costs for an average home in an average neighborhood; $5,140 in vehicle costs; $1,030 for health insurance; and income taxes of $9,386 at the 24 percent tax bracket.

That leaves $17,834, or about $1,500 per month in "discretionary income" for all other expenses, including food, clothing and utilities.

As for the 2002 family, the working parents’ combined income was $67,800. Costs included $9,000 for the mortgage, $8,000 in car expenses, $1,650 for health insurance; $9,670 for day care; and $22,435 in income taxes in the 33 percent tax bracket.

That leaves $17,045. Under this model, then, the two-working-parent family in 2002 earned $789 less than the 1970s one-earner family. That’s not a big decline -- about 4 percent in inflation-adjusted dollars over the course of close to 30 years -- but the family certainly isn’t gaining ground.

Still, there are a couple of things worth pointing out, beyond the fact that Sanders chose the wrong "d" word to describe what he was talking about.

First, Sanders was using this statistic as late as 2011, almost a decade after it was first calculated. That’s a long time for a statistic of this sort to remain unchanged.

Second, George Mason University professor Todd J. Zywicki noted in the Wall Street Journal in 2007 that the biggest increase in Warren’s comparison comes from a higher tax burden -- not from the rising cost of housing, vehicles, health insurance and day care. In fact, taxes were 2.4 times higher for the 2002 family. Each of the other items saw significant increases, but none more than doubled, as taxes did. Yet complaints about taxes holding back working families are usually a talking point for conservatives, not for progressives like Sanders.

A new talking point

At least Sanders appears to have settled on a more accurate talking point.

When we checked with his staff, they said Sanders’ usual line today is this: "Since 1999, the typical middle-class family has seen its income go down by almost $5,000 after adjusting for inflation. Incredibly, that family earned less income last year than it did 26 years ago – back in 1989."

His staff pointed to Table A-1 of the Census Bureau report on Income and Poverty in the U.S., released in 2014.

The report found that in 2013, median income was $51,939. That compares to a real median income of $56,895 in 1999, and a real median income of $52,432 in 1989. This data, we confirmed, checks out.

Even here, though, there’s more than meets the eye, Burtless said.

Burtless noted that in 1989, there were 2.65 Americans per household, while in 2013, there were 2.55. "It’s a small matter, but it means that the median income per person in the median family increased about 3 percent over that period even though the median income of households fell by 1 percent," he said.

The moral of the story: When it comes to documenting economic stagnation experienced by the middle class, it’s complicated.