A Liberal budget wouldn't be complete without a bit of social engineering. This time the tinkering – no surprise – is another scheme aimed at advancing gender equality. The idea is to make new fathers take their fair share of parental leave – something they refuse to do now.

"When we have a greater equality of parental leave and parental activities, we also are able to achieve a greater equality of life and work outcomes," Jean-Yves Duclos, the minister of families, children and social development, told the CBC.

Paternity leave was a good invention. Far more fathers take parental leave than they did 15 years ago, and family life is better for it. But all around the world, it's mothers who still take vast majority of parental leave. To increase enthusiasm among fathers, more enlightened jurisdictions – such as Sweden, Iceland and Quebec – have introduced use-it-or-lose-it leave aimed at the recalcitrant sex. Quebec now provides up to five weeks of paid leave to new fathers. The new federal law will do the same.

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This is a small but significant piece of the Trudeau government's ambitious plans to achieve gender equity. The Liberals hope that nudging fathers toward equal child-rearing will push more mothers into the work force, thus unleashing the vast economic potential that now lies fallow as they waste their time at home raising kids. More mothers putting in more paid hours would also help attain the feminist objective of narrowing the gender pay gap (which, according to study after study, is really just a child-care penalty, as a recent Vox article pointed out).

So how's it working out elsewhere?

Not as planned. Not yet, at any rate. The feminist paradise of Sweden has the longest parental leave in the world, and 90 days of leave are reserved for fathers only. But women still take more than 80 per cent of the parental leave when their first child is under 2. "It's not perfect yet," one father told the BBC. "Over the summer, for three months or something like that, feels the most common." Far more women than men are likely to work part-time, and the wage gap is stubbornly persistent. In fact, Sweden – along with other Nordic countries – has one of the most gender-segregated labour markets in the world.

Life's not perfect in Iceland either. When Iceland introduced three months of use-it-or-lose it paternity leave for fathers, the initial participation was huge – just as it's been in Quebec. But when benefits were cut, participation plunged. And the co-parenting idea never really caught on. The father "remains the mother's assistant," one researcher noted disapprovingly. As in Sweden, a shocking number of Icelandic women (one-third) stubbornly continue to prefer part-time work. Some policy makers think they have the answer. They want to shorten the official work week for everyone, so that men can shoulder more of the family duties.

But would they? Well, there's a question. A now-infamous study of parental leave in academia (where people, and leave policies, are unusually progressive) revealed the awful truth. As one headline put it, "Even in academia, dads don't do diapers." The study found that only 12 per cent of male academics took parental leave, even when it was generous and paid. On top of that, they performed "significantly less child care than their spouses." And while the mothers on leave spent the vast majority of their time engaged in intensive child care, the fathers took advantage of their time off to do research and advance their publishing agendas. The researchers ultimately concluded that universities should restrict parental leave to women, in the name of fairness.

Despite the spread of paternity leave, the issue of work-life accommodations for families remains a tremendous challenge, especially for women. But the social engineers in Ottawa are missing the boat. They persist in believing that they can re-engineer human nature so that men and women will behave alike. Along the way they make a lot of assumptions that are deeply demeaning to many women's values and choices. In the teeth of all the evidence, they assume that women's work preferences are innately the same as men's – or would be, if they had the chance. They can't imagine that a large number of women – even in the world's most advanced nations – actually like working less while the kids are young, while a great many men – despite the fact that they love their children deeply – would rather express that love by working extra hours than by changing diapers.

The Liberals' idea of family policies ignores the family unit entirely. Instead they treat Mom and Dad as isolated individuals, locked in a zero-sum battle for equality that one of them loses if he or she works less, or if one partner does less child care. Worst of all, they completely ignore the deep-seated biological sex differences that in virtually all the mammal kingdom make females more inclined to nurture – especially when their offspring are young and helpless. Hey, there! Haven't you ever heard of oxytocin?

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But to hell with role divisions. Liberals know those are just socially constructed, anyway. And so they will continue to do what they passionately believe in – try to reform society so that men and women will behave more like Liberals think they should behave. We'll see how that works out.