One of the few examples of genuine institutional prejudice against men is set to be closed this year. The Queen's Speech contained the brief announcement that:

Measures will be proposed to make parental leave more flexible so both parents may share parenting responsibilities and balance work and family commitments.

But a move towards genuine equality of parental responsibilities may prove to be a case of "be careful what you wish for" for many men, because who cares for children seems to have a strong relationship to who earns the most in society at large.

The existence of a pay gap between genders is an incontrovertible fact. The most recent in-depth study of the discrepancy, by Debra Leaker for the ONS in 2008 (pdf) found that, as of 2007, the median female wage was 11 per cent below the median male one. It's a striking figure, and made all the more relateable by the various ways in which people have presented it – none more so than the Fawcett Society, who "celebrate" No-Pay Day on October 30th each year, to represent the point at which women have done enough work to earn their salary if they were paid the equivalent of men (the discrepancy between the numbers – October 30th is only 83 per cent of the way through the year – is due to the Fawcett Society using mean rather than median salaries, and the Annual Survey of Hours and Earnings not the Labour Force Survey).

There are a lot of possible reasons for the gender pay gap, but one that is less discussed by those fighting to end it is motherhood. Indeed, there is barely a gender pay gap at all: it would be far more accurate to call it a birth pay gap.

The pay gap between women and men with no children is 8.0 per cent. The pay gap between women and men with four children is 35.5 per cent. (For one child, it's 12.3, two is 14.9, and three is 19.0).

Similarly, the pay gap between 18 and 24 year olds hovers around 1 per cent, and actually goes negative for 24 to 32 year olds. That is, the median 28-year-old woman actually earns more than the median 28-year-old man. It then rises steadily until it hits 20 per cent for over 45s:

The pay gap between men and women who are married, cohabiting or in a civil partnership is 14.5 per cent (to be clear, that is the pay gap between a woman who is married and a man who is married, not between a woman and the man she is married to); the pay gap between single men and women is -1.1 per cent. For the purposes of the point I am making, of course, one can read "single" as "unlikely to have a child any time soon".

It's not altogether surprising that having children increases the pay gap. Paid statutory maternity leave is 26 weeks; paid statutory paternity leave is two. Stepping off the career ladder for 24 weeks is always likely to hurt one's future earnings. Even the gap for childless women could be – unfortunately – explained by employers being wary of taking someone on who may then leave for six months.

All of which is to say that assigning men equal rights to parental leave may backfire if those same men are arguing for it out of a perceived sense of unfairness. There is, and always has been, a trade-off. A society which forces women to be the primary caregivers is also one which keeps men as the breadwinners. If a man wants to assume equal responsibility for looking after his child, he still finds that tricky to do (just as if a woman wants to assume an equal position in the world of business) – but the reason for that isn't a global conspiracy of feminists struggling to keep men out of their children's lives. It is the dreaded p-word: patriarchy.

End that, and men will be as free to share parental roles as we want. But if the gender pay gap equalises out, with men paying an equal share of the risk employers take on when they hire someone about to have a child and losing an equivalent chunk of career progression, we won't be the winners.