TONIGHT sees the deadline for 9,000 companies in the UK with more than 250 employees to declare their gender pay gaps.

No surprises for what this is likely to reveal. Women earn, on average, around 18 per cent less than men, according to the Equalities Commission website. The gender gap has been narrowing, is much smaller for workers under 30 years of age, and it is going into reverse for part-time work. But the overall pay gap is clearly not narrowing fast enough.

So, why has nothing been done about it? Why after nearly 50 years of the Equal Pay Act, does the problem even exist? It’s not just in the private sector. Even in the unionised public sector, women on average earn 14 per cent less than men. Even in the Scottish Parliament, which is as equality-conscious an institution as any in the UK, there was revealed a gender pay gap of 11 per cent – higher than in the BBC.

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Why can’t women just be paid the same as men for the same job? Well, in general, they are. It is illegal not to do so. There may be a few dark corners where women are illegally under paid, but these are few. The penalties are so severe that no firm or public sector bureaucracy can risk paying different rates according to gender. The deepening crisis in Glasgow City Council shows why: it is now about £1billion in the hole over its historic failure to observe Equal Pay laws.

So is it because women lack motivation and the qualifications to take on higher- paid jobs? Well, no. Women out-perform men throughout the entire educational journey, and especially at university level. Women are now 35 per cent more likely to go to university than men. Scottish universities are even looking at ways to positively discriminate in favour of male students.

So if it isn’t unequal pay, and it isn’t educational opportunity, why does the pay gap persist? Why, in many professions and jobs, do you still find men apparently doing exactly the same work as women and yet getting paid more? The answer is seniority – wage progression. Many employees get annual increments and awards for gaining qualifications. Nurses, for example, get annual pay increases “to reward you for continued service to the NHS and the continued development of your skills base”. In other words, if you stick in a job longer, and get better qualifications, you’re paid more and are more likely to be promoted. This is where women lose out.

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All the research bodies that have looked into this, like the Institute for Fiscal Studies and the Resolution Foundation, agree that the gender pay gap explodes after age 30 as many women get married and have children. They miss out in years of seniority, and they are also more likely to work part-time after they return to work. Meanwhile many men, taking on greater financial responsibility for the family home, tend to work longer hours and apply for higher-paying promoted posts.

There is still clearly prejudice against women in some senior management positions, where the old boy network persists, though this is changing. Male Hollywood actors tend to get more than women because their agents claim they are better box office, though that’s being challenged post-Weinstein. But for the vast majority of workers, the gender pay gap is largely about women starting families, taking career breaks, losing seniority and working part-time. If the gap is to be closed, this is where public policy must intervene to rectify the imbalance.

Somehow, women need to go back into full-time work earlier and apply for more promoted posts. Employers need to hold jobs open for women so that they can return at nearer the level they left, though this wouldn’t fully address the progression deficit. Firms can’t be expected to pay less experienced workers the same as time-served, skilled employees, just because they are women. This could breach the same equality legislation that gave women equal pay in the first place. Similarly, big organisations can’t sack managers just because they happen to be men.

Men should spend more time caring for their children, and equal paternity leave would help, but it is women who physically give birth, and that isn’t going to change. So, maybe we need to find ways to bridge the gender pay gap at source? It’s often said that if men gave birth, child-rearing would be seen as the most valued job in the world. That’s true. Perhaps women could get fungible credits for the time they spend with children? Wages for housework could be more than a feminist slogan.

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There could be a kind of Universal Basic Income for women (or men) who undertake child care. Why not turn managing a family into a recognised professional activity? This would prevent women becoming financially dependent when they stop work, help bridge the remuneration gap, and could even provide a recognised qualification for what is after all a highly responsible task, requiring advanced organisational and social skills. These credits could be taken into account during annual appraisals and promotion like any other certificate or qualification.

Everyone is talking UBI because automation is destroying many dull routine jobs – the ones women often do – which is a good thing. But we are going to have to re-engineer the tax system and find ways of redistributing income that don’t simply depend on people having a conventional paid job. Instead of a UBI paying people to do nothing, why not pay the people who elect to take on the most important and demanding work there is: raising the next generation? It could partly be financed by the billions that go on free child care.

Income continuity could mean that when women return to their old careers they are better qualified and more confident. A household UBI might also allow men to take on part-time work, so that both could share child care equally. Indeed, if raising children was properly paid and given professional recognition, men might start fighting harder to get a piece of the action.