For a year, British parents have effectively been engaged in a high-stakes social experiment.

The question is this: Can giving almost exactly the same rights to take parental leave to both a mother and a father—allowing them to divide the time between them as they see fit—lead to a change in the culture of mother-led parenting? And if it can, is that even desirable?

A new law, Shared Parental Leave, gives parents that puzzle to work out. Started a year ago, it holds the promise of some huge benefits. It has also exposed a host of complex fears in British society, for individuals, families, and companies.

At the core of the UK experience is a fundamental tension point: Who has the responsibility for raising society’s children? There was a time when the consensus, drowning out dissent, screamed “mothers.” That has changed. But most countries—the UK included—are still figuring out where the change leads.

Why do we need parental leave at all?

In general, governments are incentivized to ensure that the majority of their adult population, and not just men, continue in the workforce—earning, producing, paying taxes, and hopefully achieving work satisfaction in their own lives. Simultaneously, countries want to support the birth rate to the extent that those adults at least replace themselves—ensuring that productivity doesn’t fall, and there are enough people around to look after the aging population. To achieve these objectives, most societies accept, support is necessary for the years when people have young children.

Back in the day, women’s role was firmly as mothers—and domestic workers—first. With industrialization and women entering the workforce, many countries created some form of support, or at least job security, for women who wanted to return to work after having children. But maternity leave, we’ve come to realize, has its problems too—not all women want to take long career breaks, and not all types of work are easy to return to when they do. And so the case for paternity leave has grown stronger.

What we’re not agreed upon is what form that support for fatherhood should take.

In April 2015, when the UK’s then-government created Shared Parental Leave, it was radical—a switch from a system in which new mothers could take a full year of maternity leave and fathers often took just two weeks, to a system where parents can share almost a whole year of leave equally between them. But it also left a lot of problems unsolved.

What does shared leave look like?

The system now works like this: Women in Britain who have given birth to a child are required to take two weeks off work, or four if they work in a factory. But after taking that part of their maternity leave, they can now “give” any portion of their remaining 50 weeks of leave to their partner. (Same-sex couples nominate a “lead parent,” and otherwise the policy works the same, and parents adopting children or using surrogates can participate in shared parental leave after adoption leave.) It’s possible to take the weeks in separate chunks, or simultaneously.

The government ensures that employers hold parents’ jobs for them while they’re on leave and offers almost full reimbursement to employers for payments. The first six weeks, no matter who takes it, are paid at 90% of the parent’s salary. The next 33 weeks are paid at 90%, or £139.58 ($198) a week, whichever is lower. The final 13 weeks aren’t paid by the government. Companies often choose to “enhance” these statutory benefits with extra payments for part or even all of the time—though many have yet to update their maternity policy to include the fathers who became eligible a year ago.

The UK’s new sharing system is part of a conversation that’s going on across the world, not just between governments and citizens or between companies and workers, but also between parents themselves. The system is progressive compared to many parts of the world—the US and many developing countries offer very little statutory leave—but it doesn’t go as far as places such as Sweden, where leave is better-paid, more equally supported, and longer.

What the UK is attempting is a change is its parenting culture—and cultural change can be very hard, and very slow. Early evidence suggests uptake has been sluggish and hampered by obstacles. Economics, perception, and habit all stand in the way of a more equal sharing of the responsibilities of parenthood.

Already, some takeaways and tension points are emerging.

Men still earn more overall, so paternity leave is a bigger financial risk

Shared parental leave is a step in the direction of greater equality. But it also highlights other societal inequalities. The big, obvious one is financial. Because men still on average earn more, many families find that if the father takes leave, the family takes a bigger financial hit.

It’s a chicken-and-egg situation. Part of the reason women are still systematically paid less then men is because career breaks for parenting (or even the anticipation of possible parenthood) cut into their chances of advancement. Across the world, women of childbearing age report that it is hard to find work or get promotions, because of a perception that companies would be letting themselves in for years of “carrying” them—even if they have no intention of getting pregnant. Women who take long career breaks lose skills, seniority, and confidence, and find it harder to reenter the workforce, even if jobs have nominally been kept open for them.

It’s perhaps understandable that men would be reluctant to subject themselves to the same disadvantages at work. In a UK government survey (pdf) conducted before the policy came into force, 86% of men surveyed said that if SPL was available they would “definitely” or “possibly” consider taking it. But lots of them also expressed concerns. The biggest were economic, with 38% of men saying they were concerned they’d be worse off, and 32% predicting that their career would be affected by taking shared leave.

One father who took shared leave told another survey—conducted after the policy came into force—that while his organization supported the policy “on paper,” his manager made clear that he didn’t approve. “As I was the first person in our organisation to take SPL, I had to badger them to determine their policy,” the man, who is not named in the survey results, explained. His company didn’t enhance the leave, although it does so for mothers.

For some mothers, sharing parental leave is hard

It’s not just men who have some qualms about SPL. Women also report mixed feelings—specifically that for a partner to take some of the leave, they must give up a portion of their own allowance.

A survey of 1,000 employees and 200 human resources directors in the UK in the year since shared leave has been in place found that 55% of women did not or would not want to share maternity leave with a partner, according to the groups that ran it, the consultancy My Family Care, which helps companies accommodate parents, and the Women’s Business Council, which advocates for women-run businesses.

The UK’s Shared Parental Leave is a big change from the old, mother-focused system. That, in place since the 1970s, allowed women a year to parent full-time before returning to the workforce. It was itself a hard-won accommodation, and women’s groups are understandably protective of it, explained Jo Swinson, founder of Equal Power Consulting, a firm that seeks to help businesses progress toward gender parity. Before founding the consultancy, Swinson was a government minister and responsible for taking the legislation through parliament.

“The very strong feedback that we got through that consultation [was] that this should not be done at the expense of women’s rights, which I entirely understand because they have been hard-fought for,” she said.

The old system, though, perpetuated a lot of the dynamics that made a move from motherhood back into the workforce even more difficult. With hindsight, it’s easy to see why. Women with a year of leave immediately became the main carers for children, and consequently society—and their families—still saw them as “naturally” fitting that role. Many women struggle with guilt and self-recrimination, as well as harsh judgments from family and friends, for “abandoning” their children to return to work.

Holly Wilson, who worked in the fashion industry before motherhood, said she would have liked to share her leave with her husband. Wilson took a year of maternity leave for each of her children, who are now 2 and 4, getting statutory pay but no enhancement.

Her husband took two weeks in each case. “He is in a higher-paying job than me,” she explained. “So it makes sense for me to be the one who’s off.” Also, she admitted, she would never have wanted her husband to be the main carer for the children.

She has since set up her own cookware shop in London’s Stoke Newington, partly because of the flexibility that being her own boss provides.

And men aren’t supported to be full-time dads

Men, meanwhile, have historically in the UK been nudged into the role of the less-involved parent, and instead were encouraged to focus on supporting their families as breadwinners.

When his 2-year-old son was born, Ben Judd, an artist and part-time lecturer from London, took the statutory two weeks of leave from his teaching job a year before shared leave came in. At the time his wife was a PhD student so it didn’t make sense for him to take more leave from his job—even though he said the idea was “appealing.”

“Two weeks is absolutely nothing,” Judd said, but it’s not just financial considerations that prevent men from staying home longer. “I think men might rely on their careers in a different way to women, maybe, in terms of defining who they are,” he said. With regular work removed, “you start to feel like you lose your identity a bit.”

That’s a feeling not unfamiliar to mothers taking care of children full time.“It’s amazing, but also you are kind of lumbered with everything…and you kind of think: ‘Oh you get to go to work, and I have to stay here!’” Holly Wilson said.

Part of the problem may lie in how the new law is framed—giving all the leave to the mother, and allowing her to “give” some to the father. Japan had a similar law, and saw very low uptake of the option. Sweden went through a period of a similar policy, but has since changed it to mandate time off for both the mother and the father separately.

Attitudes are deeply ingrained, and tough to change, Swinson said.

“You have to recognize that there’s a journey of expectation,” she explained. “Because if you even speak to teenagers today [the girls] will already be thinking about what the shape of their careers should be, and how that might be impacted by having a family…Speak to teenage boys; it’s not even on their radars.”

To get fathers truly involved in the project of child-rearing, it’s becoming clear that they need societal support, just as mothers do. One major way to signal this support, and to start changing perceptions, is to ensure paternity leave makes so much financial sense that men take it despite other worries, like how they might be seen, or whether their careers might be affected.

Working Families, a charity that supports people to combine work and family, found that in the first six months after the UK introduced shared leave, only between 0.5% and 2% of eligible fathers used it. Most companies expected that figure to increase; but the policy still might not go far enough.

Since 1995, Sweden has set aside a portion of its parental leave for dads, extending the portion of the leave that only the father can take to 90 days this year (the total is 480 days, or 68 weeks). If the man in a partnership doesn’t take the leave, the family loses it. All leave is paid at 80% of the person’s salary, negating to a large extent the financial disadvantages of being off work.

It only works if companies buy in

If the UK’s shared policy is to work at all, companies will have to get on board. Most firms offer some form of enhancement for “statutory” maternity leave, which only pays a fraction of what many people earn. So far, though, many are not extending that to fathers. Nearly half of companies surveyed by My Family Care said they didn’t enhance paternity leave in any way, and of those, 50% were waiting to see what the appetite would be before making a decision.

Company attitudes increasingly matter—especially in places without much state support. A notable absence from the roster of countries that support parents in the first months of a child’s life is America, where only a few states have introduced systemic support.

More and more, the lack of structured help for new parents is being challenged and subverted by private companies, especially when it comes to paternity. Netflix announced in August that it would offer a full year of paid leave to parents—mothers or fathers—who had a child or adopted one. Salesforce in the US offers eight weeks of paid leave for mothers and fathers alike. Facebook offers mothers and fathers the same paid four months. Notably, these are all technology companies, and all appear keen both to promote a supportive, holistic culture, and to get a competitive edge on hiring, and retaining, people. In some sectors, parental leave is becoming the new hot, must-have perk that gives hirers an edge.

Putting the option in place for fathers to take leave is only half the battle. Even when it’s there, there are barriers to taking it. Companies can take a lead on helping to change that sticky culture—it they want to.

Companies that have done so report significant benefits. When shared leave came into force in the UK, Citigroup made the decision to treat it, and the benefits associated, in exactly the same way as maternity. They have been “bowled over” by the uptake, according to Xanic Jones, who specializes in diversity and inclusion for the group’s human resources department.

Double the number of employees took shared leave in the first year than the bank predicted, she said. These dads may be the “pioneers” of taking shared leave, but the process of “normalizing [it] into Citi’s culture,” has begun, she said. The group is “recognizing that it is OK for both mums and dads to take a longer period of time off work when a new baby arrives into their family and that our staff will be more loyal, more productive when they return and flourish in their careers as a consequence.”

Will the UK go the way of Sweden? Right now, the trajectory is more tangential, with the current Conservative government proposing to extend shared leave to grandparents. Whether that’s a recognition of the importance of extended family or a get-out clause for “busy” dads remains to be seen.