While its not a universal construct - it is challenging to look at the history of many cultures who arrived at a similar distribution of labor and the behavior of other mammals and not wonder if men and women aren't inherently different in some generalized way that isn't readily alterable.

For those of us on the other side of the debate, there is always this vague sense of some people are underestimating the impact of these intrinsic differences.

Lots of things were/are found in many unrelated human societies, such as slavery, which was found all over the world for thousands of years and still exists in many places/forms today. Something being common isn’t an indication of whether or not it is intrinsic or “the only way”. We only make progress by challenging social structures and trying to improve them. Perhaps these kinds of commonalities exist all over because they are effective at controlling/enforcing oppression and power.

My point isn’t that men and women aren’t at all different, it’s that institutions use the “it’s human nature and it can’t be changed” argument to hide oppressive/coercive practices that are avoidable and able to be removed.

> For those of us on the other side of the debate, there is always this vague sense of some people are underestimating the impact of these intrinsic differences.

Nailed it.

I suspect that this distribution of labor is not due to intrinsic differences, but came with the invention of agriculture, and what we are dealing with now is just that we are used to these structures. Hunter gatherers did something else, and now that we are not all subsistence farmers we can do something else again.

> So why are women still expected to be caretakers of toddlers, middle-schoolers, and tweens? Surely, there isn’t a physiological reason.

Enhanced emotional bonding due to emissions of oxytocin during breasfeeding? Make men sit with children several times a day while getting oxytocin shots and perhaps they would have much stronger bonds with their children.

From Wikipedia: Maternal behavior: Female rats given oxytocin antagonists after giving birth do not exhibit typical maternal behavior. By contrast, virgin female sheep show maternal behavior toward foreign lambs upon cerebrospinal fluid infusion of oxytocin, which they would not do otherwise.

Of course not. But the average woman wants three kids. WHO says babies should breastfeed for two years. My daughter has nursed for 13 months. A mother taking off three chunks of 13 month leaves while married to a man probably earning as much or more than she does (since women disproportionately marry similar or higher earning men) is going to -- at some point -- realize that it's easier to stay at home. That is exactly what my wife did.

> Even the concept of the nuclear family - with one possible nurse and mother figure - isn’t a universal construct of human societies.

Yes it is. The only people who claim this are people who've never lived in an extendend family situation. You still know who your mom and dad are. They are still the primary care givers, and babies still have a primary attachment to their moms when born that self-reinforces itself unless there is some major disruption.