(Bloomberg Opinion) -- The coronavirus seems to have overtaken every aspect of our lives, from the mundane task of buying toilet paper to the very pressing question of when your spin class will resume. Now it’s even delaying your tax deadline.

That bill from Uncle Sam will come eventually, though, which means over the next few weeks — or months — Americans and their accountants will be busy trying to squeeze every last dime out of their refunds and taking stock of household expenses.

For many two-career couples, this annual budgeting exercise can become pretty dispiriting. Once you tally up the costs of nannies, daycare, commuting, dry cleaning, housekeeping, meals out and other work-related expenses, the marginal benefit of an additional salary can shrink close to zero. If one partner decides to quit, it’s the one with less potential earning power: often a wife.

It may be tempting to use this grim calculus to renew calls for subsidized childcare and more generous parental leave. But a recent staff discussion note from the International Monetary Fund found that there’s a bigger swing factor: the way we file our taxes.

Switching to a system where everyone files separately, whether or not you’re married, would lead to a 15.5 percentage point jump in women’s labor-force participation, the IMF found, compared with a 1.6 percentage point rise for instituting 18 weeks of paid maternity leave and 4.8 percentage points for halving the cost of childcare.(1) That would amount to adding more than 20 million women to the U.S. workforce, based on back-of-the-envelope calculations.(2)

To understand why, take a look at this striking example from Edward McCaffery, a professor at the University of Southern California Gould School of Law and author of “Taxing Women.” He’s outlined a hypothetical scenario of a family where the husband (let’s call him Joe) makes $60,000 a year and the wife (let’s call her Mary) stays home with their two kids. Mary has been offered a job for $30,000 a year. However, once you factor in federal income taxes, social security, state and local taxes, not to mention the related expenses of going back to work, Joe and Mary would end up with just a net $1,000 increase in their joint income.

This comes down to what’s called the secondary-earner bias. Mary’s first dollar doesn’t start getting taxed at $0, as Joe’s would, but rather at $60,001, because joint filers are recognized as one unit of income. Thanks to the U.S.’s progressive marginal rates, successive portions of income get taxed at higher levels — the first bracket starts at 10%, the next at 12%, the following at 22%, and so on. The system is problematic for spouses who are “on the margin” about working, as McCaffery puts it. Seen this way, it’s easy to understand why Mary would get discouraged.

The math is even more deflating for a woman whose spouse makes, say, half-a-million dollars. She’d start getting taxed at $500,001, which could push her into the highest bracket right off the bat. We can’t exactly say she’s hard up with that handsome family income— but if she opts to stay home because her salary gets gobbled up by taxes, that costs the workforce a talented person. Multiply that out across all the women on the margin and you get a sense of why this invisible problem has such a big impact. The International Labour Organization found that narrowing the gender gap in participation by 25% in five years could boost global gross domestic product by 3.9%.

McCaffery’s book published more than two decades ago, yet the secondary-earner bias hasn’t gone away — which makes you wonder where joint filing came from and why the U.S. has stuck with it all these years.

Federal income taxes have existed since 1913. At that point, everyone filed separately. The tax was highly progressive; for the 1918 tax year, the rate schedule had no less than 49 brackets. This created an incentive to spread income to family members. For example, a man earning $10,000 in 1930 would pay 6% in taxes, or $600, while two individuals earning $5,000 would pay 3% each, for a total of $300. Rich men, in particular, got pretty clever about moving income to their wives.

That same year, the Supreme Court ruled that this practice was legal in the states entitling wives to half their husband’s assets acquired after marriage, so-called communal property states. This ended up creating a massive discrepancy: Couples in other states were paying 40% more in federal taxes, according to H&R Block Inc. Partly to address this, Congress introduced joint filing in 1948.

It wasn’t until the 1990s, with the publication of McCaffery’s book, that the public began to worry this system was keeping women like Mary out of the workforce. (Until that point, the debate circulated almost exclusively among policy wonks.) Yet proposals to move toward separate filing met stiff resistance. One of the loudest critics was conservative lioness Phyllis Schlafly, who argued that there’s no such thing as gender neutral tax policy and sought to protect the traditional one-earner household. Another argument was that taxing the family as a unit more closely reflects patterns of household consumption. Though advocacy for separate filing eventually lost momentum, the book “The Two-Income Trap,” by Amelia Warren Tyagi and Elizabeth Warren, recognizes the struggles of dual-earners.

To clear up any confusion, “married filing separately” isn’t the answer. Such couples often go into a higher tax bracket and miss out on certain benefits, or those perks get phased out, according to Jackie Perlman, Principal Tax Research Analyst from H&R Block’s Tax Institute. That’s why most people file jointly: Of the 153 million tax returns filed for the 2017 tax year, 55 million were joint filers(3) and just 3.2 million were married filing separately.

A sensible, centrist way forward, then, would be to make separate filing optional for all taxpayers. The U.S. is in a small minority of countries where married taxpayers still file jointly. Those that once followed the American model slowly began to shift to separate filing as low female labor-force participation became too costly, McCaffery has written. In Sweden, for example, research shows that the sharp increase in working married women came as a result of the individual tax reform of 1971. Employment in this demographic would have been 10 percentage points lower in 1975 if the 1969 statutory income tax system had still been in place then, the study found.

No one likes to think about taxes. It’s a lot easier to get excited about issues more of us understand, like subsidized childcare and longer parental leave. But if we’re going to cheer the lengthening list of companies that are coming to the table — at long last — with family-friendly policies, it’s worth educating ourselves about the issues that will make a bigger difference.

(1) The IMF analysis applies to middle-class working mothers of preschool children. It reduces the cost of childcare per child to 5% of family income from 10%, and excludes some women who benefit from federal programs.

(2) Using data from the Bureau of Labor Statistics.

(3) This includes surviving spouses.

This column does not necessarily reflect the opinion of Bloomberg LP and its owners.

Rachel Rosenthal is an editor with Bloomberg Opinion. Previously, she was a markets reporter and editor at the Wall Street Journal in Hong Kong.

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