Farmers accepted marriages between young women and older men because they have more tolerance for power differences and less expectation of equality.
Or: Such asymmetric relationships worked well in farmer economies, where more established men had the resources to support (often several) wives, while younger men simply hadn't had the time and success yet to do so. In forager economies individuals couldn't accumulate so much private property as to make that difference. Farmer societies that developed an ideology that would support their economic success would outcompete those that didn't.
(This approach is somewhat Marxian, I'll admit: expecting that societies will develop an ideology that matches their economy. But it makes more sense to me than the opposite: expecting that cultural norms drive the creation of particular economic institutions.)
I don't see this as being contrary to what I said. I agree that large power differences made sense for the economies of farming cultures, and that is why cultural norms shifted toward justifying such power differences. But I think the reason things shifted back is that rich industry-era people decided to move back toward the more egalitarian relationships was because it felt more natural (we may have evolved some adaptations that made us more successful farmers, but the primitive egalitarian urges are still strong, and we feel better having switched back).
I tend to agree with the Base/Superstructure concept from Marxian theory, although I'm generally skeptical about Marx's methods of deriving his theories.
Why don’t men go for younger women? That’s not quite accurate, because they do, but we seem to have a cultural norm against girls who have recently exited puberty, even though they are highly fertile. Some time in the last few hundred years, we reached a cultural norm that as a man gets older, it’s increasingly less appropriate for him to be with a sixteen year old. That’s not long enough to override thousands of years of evolution, so what could have contributed to it?
The best solution I’ve heard started by looking at who benefits from this norm [older women] and wondering whether they could have contributed to it. After all, the strength of this norm has been increased in the last sixty years or so, which coincides with the period in which women’s power has increased.
One alternative I heard recently is that it doesn’t make sense biologically to go for women who are recently post-pubescent, as they don’t make the best mothers. Instead, a slightly older woman [mid-twenties] makes sense to be the best mother. This is perfectly plausible.
Another, less plausible, suggestion I’ve heard is that it’s to do with mental capacity. I find this unconvincing because we have few objections to a high-status man dating a beautiful but low-intellect woman.
Thoughts?