Last week in the U.K.’s House of Commons, the governing party appeared to cast aside its “pairing rule,” a chamber practice that has been used to preserve voting rights of absent Members of Parliament (MPs) — including MPs out on maternity leave.

This caused a huge outcry, with the minister for the Cabinet Office forced to respond to an urgent question in the House of Commons this week, promising that such errors would not be repeated. But the ensuing controversy also highlights the limitations of relying on informal legislative practices in increasingly diverse parliamentary bodies.

This is how legislative pairing works

Pairing is an informal arrangement between two legislative parties when an MP is sick, on parental leave, or otherwise not present to vote. If a legislator knows he or she will be absent, the party whips can “pair” the absent MP with an opposing party MP. Here’s the key: The opposing party MP promises not to vote. By refraining from voting, the votes of the paired legislators cancel the other one out.

With pairing, unavoidable absences do not penalize the missing MP’s party. Moreover, according to Lord Michael Heseltine, who has held positions in U.K. politics since 1966, legislative pairing prevents the “spectacle” of ambulances arriving at Parliament to bring ill politicians to vote.

This is informal practice, and the House of Commons does not officially recognize paired voting. Instead, MPs or party whips make these arrangements at the party level, as needed. For pairs to function as intended, both whips and politicians must honestly adhere to agreed arrangements. The legislatures of Canada, Australia and the United States all use versions of paired voting procedures.

The system broke down in the House of Commons

The pairing practice applies to all MPs — including men on paternity leave. But it disproportionately affects female MPs such as Jo Swinson, a Liberal Democrat on maternity leave after the birth of her second child. Here’s what happened last week: A Conservative MP, Brandon Lewis, cast two votes that violated a pairing agreement that had been set up to accommodate Swinson’s maternity leave.

Conservatives at first defended the votes Lewis cast as an “honest mistake” and a “genuine mix-up.” Other Conservative MPs later conceded that their party whip asked MPs to break their pairing arrangements on two Brexit amendment votes because of anticipated close margins. As it turns out, Lewis’s votes did not ultimately change the outcomes.

The pairing practice was also called into question in June, when Labour MP Naz Shah was denied a pair — and then had to leave the hospital to vote on the “meaningful vote” amendment to Brexit. Shah, who was on morphine at the time, had to be wheeled into Parliament in her pajamas, carrying a sick bucket as a precaution.

There are more durable ways to solve the problem

Many MPs reacted to the violation of Swinson’s pair by calling for the resignation of the Conservative party whip; others called for change. Indeed, some MPs describe current voting protocols as “archaic and undignified,” part of an “18th century model of work” created when women did not have the right to vote or stand in Parliament.

Liberal Democrat MPs Swinson and Christine Jardine, as well as Harriet Harman of the Labour Party, took the opportunity to renew long-standing calls to overhaul the pairing system. Harman, who served as the first Minister for Women in 1997, said, “Why should that constituency lose the right for the vote in their name to be cast because their MP is having a baby?”

Among 43 potential changes, the 2016 Good Parliament Report — supported by the House of Commons Speaker John Bercow and written by Birkbeck University professor Sarah Childs —  proposed proxy voting for absent MPs as an alternative to pairing. Proxy voting allows MPs to delegate their vote when they are absent to an MP who is present.

Unlike pair voting, proxy voting lets an MP continue to represent his or her constituents without worrying that an arrangement will be broken for a particularly important vote. Despite significant support for proxy voting, and a cross-party report detailing how it could be implemented, parliamentarians postponed a scheduled debate on proxy voting a few weeks ago.

These sorts of changes happen slowly. Here’s an example: One such adaptation came 10 years ago when, after 20 years of campaigning, Parliament replaced one of many on-site bars with a nursery for the children of politicians and staff.

These may seem like small obstacles. But as political theorist Nirmal Puwar says in her book “Space Invaders,” cumulatively, even small barriers can make women feel unwelcome and discourage underrepresented individuals from running for or remaining in office. Even now that women are one-third of all MPs, Parliament has never formally incorporated maternity leave into the chamber rules.

The U.S. Congress also struggles to accommodate women legislators

The diversification of legislatures has generated structural changes elsewhere in the world. In the United States, for example, female senators had to fight for adequate bathroom facilities in 2007. And in an especially striking sign of the male-dominated institution, women had to lobby for access to the Senate swimming pool, which seemingly barred women because two male senators liked to swim naked.

This year, Tammy Duckworth became the first sitting U.S. senator to give birth while in office. The birth of Duckworth’s daughter prompted a rule change to allow babies on the Senate floor.

New Zealand Prime Minister Jacinda Ardern also made news as the second world leader in the modern era to have a child in office. The New Zealand parliament expanded its rules to permit MPs’ children into the chamber, suggesting that the diversification of a country’s political leaders can generate procedural changes to support the democratic process.

Perhaps Jo Swinson’s maternity leave debacle and the ensuing lack of trust in the pairing system will lead to permanent institutional solutions to ensure that the voting rights of MPs on parental or sick leave are not unduly harmed by legislative majorities in pursuit of their agendas.

Rebecca Kuperberg is a PhD candidate at Rutgers University, studying women and politics.

Mary Nugent is a PhD candidate at Rutgers University, studying women and politics. Follow her on Twitter @marynugent1.