You’re My No. 1. Except on Our Joint Tax Return.

Do men open the door and politely let their wives go first? Or do they charge ahead?

Guys, it’s the latter.

According to a first-of-its-kind assessment from researchers from the U.S. Treasury Department and the University of Michigan, men’s names were listed first on 88% of joint returns filed by opposite-sex married couples in 2020. That figure has trickled down a little since 1996, when nearly all returns—97%—listed the man’s name first.

The gender-equality movement of the past few decades changed the composition of boardrooms, universities, operating rooms and legislatures. It has barely budged the man-goes-first convention on the 1040. Among couples filing jointly for the first time in 2020, 76% put the man’s name first.

There is now someone in America with the actual title of Second Gentleman. That is Douglas Emhoff, a lawyer who is married to Vice President

Kamala Harris,

and who slowed his career down after her election. Yet on their tax return, the Second Gentleman is Number One, while Ms. Harris is effectively Vice Taxpayer. (The vice president’s office didn’t respond to a request for comment.)

A few other 2020 presidential candidates seem like the exemptions that prove the rule. Sen. Kirsten Gillibrand (D., N.Y.), Sen. Amy Klobuchar (D., Minn.) and former Montana Gov. Steve Bullock disclosed joint tax returns during the campaign with women listed first.

In the overwhelming number of cases, however, the bottom line is that men go on the top line.

“I’m in the South, so probably 99% of the time, the default is to put the husband first on the return,” said Jan Lewis, an accountant in Jackson, Miss.

A client once got upset, she said, because their correspondence referred to “Taxpayer” and “Spouse,” when they are both spouses.

“I thought well, OK, she has a point,” Ms. Lewis said. “But legally, in the eyes of the IRS, it’s Taxpayer and Spouse.”

Ms. Lewis, whose name is second to her husband’s on her own tax returns, does have to remember to start with her spouse’s name when making her own quarterly estimated tax payments to the IRS.

That’s because bureaucratic inertia is strong, too. Taking turns might work for loading the dishwasher or clipping the baby’s toenails, but it’s a mess for tax filing. The Internal Revenue Service instructions for Form 1040 include a tip for taxpayers: If you are filing a joint return with the same spouse as last year, put your names and Social Security numbers in the same order as last year.

It isn’t a requirement, per se, just some friendly neighborhood advice from the government to ward off disaster, like when they tell you to carry bear spray at Yellowstone National Park. If you’ve paid any attention to IRS backlogs and difficulties processing mountains of paper over the past few years, you definitely don’t want to do anything that might risk your tax return being lost in the agency’s labyrinth of sadness.

So a barely considered decision made early in a marriage for whatever reason—who was making more money at the time, who managed the household finances, who prepared the taxes, the accountant’s default setting—gets stuck in place. Fewer than 2% of married couples switch who goes first from year to year.

Marilyn Holtz Patti, who runs an art business in western Massachusetts with her husband of 48 years, Tom Patti, said she’s always gone second on the tax forms—though she has no idea how that came about. She says she owns 51% of their business.

In an interview Tuesday, the couple bantered about the origins of the order.

“Maybe because he’s older?” she said, laughing.

Mr. Patti cracked that maybe his wife had put him first to make him more visible to the government. “If they come after somebody, I’ll be the first to go,” he joked. “She’s the smart one.”

(Of course, that isn’t how IRS enforcement actually works.)

A tax return isn’t written correspondence where the person doing the writing signs first, so there is not even an etiquette rule to follow for newlyweds trying to decide the right order, said Lizzie Post of the Emily Post Institute.

“You could flip a coin,” she said. (Another government tip: Never mail paper money or metal coins to an IRS office.)

Michigan economics professor Joel Slemrod and his colleagues had been working on broader research about gender biases in the tax system when they realized they had enough information to generate some data about whose name goes first. “I learned pretty quickly that nobody knows—and that’s the kind of thing I like,” he said.

Researchers found that women are more likely to be listed first in returns filed by younger people. Men’s chances of being listed first increase along with their share of the couple’s wages.

Woman-first tax returns are most common in the District of Columbia, where fewer than 80% of opposite-sex married couples list the man’s name first. Among states, Vermont, Oregon, Maine and Alaska are the only places with woman-first rates above 15%.

At the other end are three states where man-first returns exceed 90%: Iowa, New Jersey and Utah.

Among same-sex married couples, higher-earning and older spouses are more likely to be listed first.

The study found that man-first returns in opposite-sex couples tend to include riskier investments and be associated with higher rates of noncompliance with the tax code. That finding is consistent with prior personal-finance research about risk-taking by gender. Man-first returns also include larger charitable donations.

Mr. Slemrod, the Michigan researcher, has been married for 42 years.

“I am primary. Always have been. And it is also true that I am mostly in charge of the finances, and I 100% do the taxes,” he said. “I never considered switching.”

Write to Richard Rubin at richard.rubin@wsj.com

Do men open the door and politely let their wives go first? Or do they charge ahead?

Guys, it’s the latter.

According to a first-of-its-kind assessment from researchers from the U.S. Treasury Department and the University of Michigan, men’s names were listed first on 88% of joint returns filed by opposite-sex married couples in 2020. That figure has trickled down a little since 1996, when nearly all returns—97%—listed the man’s name first.

The gender-equality movement of the past few decades changed the composition of boardrooms, universities, operating rooms and legislatures. It has barely budged the man-goes-first convention on the 1040. Among couples filing jointly for the first time in 2020, 76% put the man’s name first.

There is now someone in America with the actual title of Second Gentleman. That is Douglas Emhoff, a lawyer who is married to Vice President

Kamala Harris,

and who slowed his career down after her election. Yet on their tax return, the Second Gentleman is Number One, while Ms. Harris is effectively Vice Taxpayer. (The vice president’s office didn’t respond to a request for comment.)

A few other 2020 presidential candidates seem like the exemptions that prove the rule. Sen. Kirsten Gillibrand (D., N.Y.), Sen. Amy Klobuchar (D., Minn.) and former Montana Gov. Steve Bullock disclosed joint tax returns during the campaign with women listed first.

In the overwhelming number of cases, however, the bottom line is that men go on the top line.

“I’m in the South, so probably 99% of the time, the default is to put the husband first on the return,” said Jan Lewis, an accountant in Jackson, Miss.

A client once got upset, she said, because their correspondence referred to “Taxpayer” and “Spouse,” when they are both spouses.

“I thought well, OK, she has a point,” Ms. Lewis said. “But legally, in the eyes of the IRS, it’s Taxpayer and Spouse.”

Ms. Lewis, whose name is second to her husband’s on her own tax returns, does have to remember to start with her spouse’s name when making her own quarterly estimated tax payments to the IRS.

That’s because bureaucratic inertia is strong, too. Taking turns might work for loading the dishwasher or clipping the baby’s toenails, but it’s a mess for tax filing. The Internal Revenue Service instructions for Form 1040 include a tip for taxpayers: If you are filing a joint return with the same spouse as last year, put your names and Social Security numbers in the same order as last year.

It isn’t a requirement, per se, just some friendly neighborhood advice from the government to ward off disaster, like when they tell you to carry bear spray at Yellowstone National Park. If you’ve paid any attention to IRS backlogs and difficulties processing mountains of paper over the past few years, you definitely don’t want to do anything that might risk your tax return being lost in the agency’s labyrinth of sadness.

So a barely considered decision made early in a marriage for whatever reason—who was making more money at the time, who managed the household finances, who prepared the taxes, the accountant’s default setting—gets stuck in place. Fewer than 2% of married couples switch who goes first from year to year.

Marilyn Holtz Patti, who runs an art business in western Massachusetts with her husband of 48 years, Tom Patti, said she’s always gone second on the tax forms—though she has no idea how that came about. She says she owns 51% of their business.

In an interview Tuesday, the couple bantered about the origins of the order.

“Maybe because he’s older?” she said, laughing.

Mr. Patti cracked that maybe his wife had put him first to make him more visible to the government. “If they come after somebody, I’ll be the first to go,” he joked. “She’s the smart one.”

(Of course, that isn’t how IRS enforcement actually works.)

A tax return isn’t written correspondence where the person doing the writing signs first, so there is not even an etiquette rule to follow for newlyweds trying to decide the right order, said Lizzie Post of the Emily Post Institute.

“You could flip a coin,” she said. (Another government tip: Never mail paper money or metal coins to an IRS office.)

Michigan economics professor Joel Slemrod and his colleagues had been working on broader research about gender biases in the tax system when they realized they had enough information to generate some data about whose name goes first. “I learned pretty quickly that nobody knows—and that’s the kind of thing I like,” he said.

Researchers found that women are more likely to be listed first in returns filed by younger people. Men’s chances of being listed first increase along with their share of the couple’s wages.

Woman-first tax returns are most common in the District of Columbia, where fewer than 80% of opposite-sex married couples list the man’s name first. Among states, Vermont, Oregon, Maine and Alaska are the only places with woman-first rates above 15%.

At the other end are three states where man-first returns exceed 90%: Iowa, New Jersey and Utah.

Among same-sex married couples, higher-earning and older spouses are more likely to be listed first.

The study found that man-first returns in opposite-sex couples tend to include riskier investments and be associated with higher rates of noncompliance with the tax code. That finding is consistent with prior personal-finance research about risk-taking by gender. Man-first returns also include larger charitable donations.

Mr. Slemrod, the Michigan researcher, has been married for 42 years.

“I am primary. Always have been. And it is also true that I am mostly in charge of the finances, and I 100% do the taxes,” he said. “I never considered switching.”

Write to Richard Rubin at richard.rubin@wsj.com