The most transformative campaign promise of this election was made on Friday. And it came from the Lib Dems.

The offer to increase paternity leave to a month has the potential to be huge.

Its impact alone would be impressive but its full power lies in setting a direction of travel towards more paternal involvement in childcare. That is something that could improve the lives of women, men, children and society as a whole.

Labour are expected to make a similar pledge when their manifesto is officially unveiled. If Theresa May really believes in evidence based policy making, if the Tories really believe in setting people free to make the best decisions for their own lives they’ll follow suit.

For the evidence is staggering. The headline figure, quoted by Jo Swinson – former Coalition equlities minister and architect of the Lib Dem announcement on paternity leave – in a well received TED talk, is that dads make kids smarter. Really. The most telling indicator of academic attainment is not money, it’s paternal engagement. Cleverer kids are a good end in themselves, but of course a better educated society also tends to be better off.

Instead of going on about grammar schools, the evidence for which is shoogly at best, the government ought to be focussing on fathers.

As well as criticising cuts to education funding, Labour ought to be putting parental leave front and centre. Not least because as a policy it shouldn’t work out too expensive.

And there’s a host of other benefits. Fathers that are more involved enjoy better relationships with their partner and children and better mental health. Those men’s rights activists who claim they hate feminism because male suicide will surely be cheering Jo Swinson to the rafters today. (Unless of course they actually hate feminism for some other reason…)

Men who look after their children more are changed physically. They live longer. Research has shown that childcare changes women’s brains. Surprise, surprise, if men do the childcare, their brains – what with being from the exact same species as women – change in the same way whether that be the memory loss of "baby brain" or the increase in empathy that comes from looking into your progeny’s eyes.

Children who have more paternal input to their upbringing have higher self-esteem, better friendships and are less likely to tangle with the law in their teens. The boys are more inclined towards gender equality, the girls feel more empowered. These are the well-balanced children the rest of us hated when we were in our teens because we wanted to be them. Through parental leave, politicians have a chance to create more of them.

Of course not all families can or do have a dad to get involved. But most do, and in most cases he’s not as engaged as the mother.

That’s down to economic and social factors.

Yes, there’s shared parental leave these days – a policy piloted into law by, her again, Jo Swinson. Take up remains relatively low, at around 5 per cent. Yet, when polled, most men say they want parenting to be shared equally. The kind of people who say the lack of parents using shared parental leave shows it’s not popular would presumably argue that you don’t see many Ferraris on the road because most folk prefer their Ford Focus.

Currently paternity leave can be expensive. Men get two weeks at home by law. But parental leave – the bit that can last up to a year and that is overwhelmingly used by women - only attracts a standard pay of £140 per week. As long as there is a gender pay gap the cost to the household of the man taking time off is always going to be the higher relatively. The next step in evolving shared parental leave will be to look at making it better paid.

And then there is the social side. While it’s still regarded as OK, normal even, for a woman to take months off after having a baby and to request a flexible working arrangement upon her return to the workplace, it is different for men.

To men, going part time may feel like career suicide – and even if in the event it’s not the fear that it will be puts many off asking - and yet we expect it of women. And that sums up the societal part of the problem.

Men are put off childcare because it is portrayed as women’s work. And boys are brought up to disdain the female realm. Just think of a boy’s reaction to being asked to wear pink versus a girl’s reaction to being asked to wear blue.

I, along with my partner, recorded all the ways that boys and girls are gendered and sent down separate paths in our book The Gender Agenda. These cues from society, family, media all put limits on their worldview and the opportunities they feel fit to take because of their gender.

Changing those ingrained attitudes will be hard. Government can speed things along. If men find it awkward to ask for shared parental leave, give them paternity leave. The confidence that comes from a month at home with a newborn might embolden men to want more and encourage their partners to demand more when it becomes clear that what’s in a parent’s pants has no bearing on their ability to fasten a nappy.

It’s the Conservatives that are supposed to be keen on equality of opportunity. There won’t be equality until men and women are free from the economic and cultural constraints that prevent them deciding how to run their own family as they see fit. More paternity leave is a good place to start.