We have a problem for Equal Pay Day, which comes on Tuesday, April 2, this year. The problem is that no one seems to be grasping that we're talking about equal pay among human beings. Human beings not being equal, something that we might consider important to note when doing science, rather than politics or propaganda at least.

The importance here is that different people have different desires and propensities in the way they wish to spend their life. There are even group differences in these desires and propensities — although we must understand that when we talk about such groups we are therefore talking about probabilities — on average, a group has a characteristic, but that does not mean that all members do, nor that none outside the group have it.

To the details, as Mark J. Perry explains that we have no evidence at all that men and women are paid different amounts to do the same job. What we have is evidence that men and women have different earnings on average. These are not the same thing.

To pay someone differently for the same job based upon some protected characteristic, gender, sexuality, race perhaps, is illegal and has been for some decades. In a society as litigious as the United States, if it were happening on anything above the most minuscule scale, the courts would have no time to rule on anything else. We don't see those cases, the plaintiff bar isn't making a fortune, so the best assumption is that different pay for the same job isn't happening.

We do, though, see different propensities. From Perry again, men suffer the vast majority of work-related fatalities . This could just be because men are idiots and every male life has contained moments where that's true. It's generally not women, for example, saying, “Hold my beer and watch this.” But we do also know that men are less risk averse than women. Men are more likely to be willing, therefore, to trade risk for pay. Some jobs are more dangerous and they gain higher pay as a result.

That original claim that women make only 77 cents on the male dollar, or whatever variation of it gets pushed at us, therefore needs to be filtered through that risk difference. A number of other filters are necessary too: hours spent at work in a week, years spent in the workforce, education levels, and on and on. When we do crunch through all of those filters, we find very little difference in pay.

We still, obviously enough, see the difference in average earnings. But there's still an explanation for that difference.

One possibility is that some feminists are right and we're just in a patriarchy. Everyone gangs up on women and that's that. The problem here is that even when we look at subsystems of the economy where bias or favoritism aren't remotely possible, when neither the pay nor the job are gendered in any manner whatsoever, we still see an earnings gap. There is simply nothing in the Uber system that allows consumers, management, nor the patriarchy to create a gender difference in wages. Yet we still, as we've noted before , have a 7% earnings gap between men and women.

The only answer that makes any sense is to agree that we're dealing with human beings here. As a sexually dimorphic species, we should not be surprised to find a gender-based difference in propensities and desires. When we do look through this lens, we find our little secret: Men and women tend to react differently to the arrival of children. That then explains all of the other things we can observe.

Lesbians don't have an earnings gap with men . Single, childless, never-married women don't have an earnings gap with men . Many more women interrupt careers to take care of children than do men. This explains why there are fewer women at the very top of the earnings distribution, the place it takes decades of dedicated effort to reach.

It is this, this one thing, which explains all we know on this subject. We don't have a gender pay nor earnings gap, we have a motherhood or child care gap. On average and in general, more women take on child care, while more men work straight through their children's gorgeous youths.

This is something we're not hugely surprised about in a sexually dimorphic species. Even if it does surprise, it tells us what the only viable solution is going to be. Around and about when men, on average and in general, do just as much putting career aside to parent their children, then and only then will there no longer be a gender pay gap.

Some years ago I put this logic forward in my native Britain and it caught the political eye. The country now has paid paternity leave as a consequence, I'll claim only a modest portion of the blame, but some. This will indeed aid in reducing that gap. But there's still that terrible sneaking suspicion that this won't be enough, partly because the take-up rate is very low, but mostly for a very different reason.

Think through this again. Assume even that I'm right, the gender pay gap is a consequence of, and only a result of, a generally and on average difference between men and women in the desire to care for young children. More women would like to do this than men. From that, all else flows. So, when do we think this preference is going to end? For it's at that exact same time that the gender pay gap is. My own estimation is that the preference never will change, which is a bit of a problem for any attempt to close that gender pay gap, isn't it?

Sure, maybe I'm just a patriarchal cisgendered heteronormative doing some mansplaining. Or maybe human beings just are a sexually dimorphic species.

Tim Worstall (@worstall) is a contributor to the Washington Examiner's Beltway Confidential blog. He is a senior fellow at the Adam Smith Institute. You can read all his pieces at The Continental Telegraph.