State of the Union reading level over time has declined quite a lot.
Written sentences are getting shorter and less recursive over time: 1, 2, 3, 4.
Heh. This is a good observation.
Could it just be increasing literacy? One hypothesis might be that as more people read at all, the average reading level drops.
I don't think the timelines match up super well for this. My sense is that Eliezer at least saw a peak in scientific progress around the early 20th century, which none of these metrics seem to correspond to, so I don't really feel like a decline is that indicative of a confirmation of the relevant model.
Bowling Alone is the canonical reference for the shift in memetic transmission medium, thus changing selection pressures.
One thing that really stuck with me (no cite unfortunately) was this author I once ran across writing in the late 70's or early 80's who noted that something really bad had seemed to happen with the transmission of metis from the greatest generation to the boomers. Like a generation just forgot to actually raise their kids (insert rabbit hole on public education here) and just assumed that transmission happened automatically. Reading older material (heavy selection filter so...) I also often can't shake the feeling that moderns are sort of...feral is the term that really fits. Back to the Bowling Alone thesis, if our peers are what socialize us, then it's the loss of the medium for the formation of strong social bonds in early adulthood. It might ring particularly strongly for me since I participated in a relative anachronism with the same purpose: Boy Scouts, and had the experience of feeling like it was completely insane that the experiences in Boy Scouts were optional. Many fathers are quite bad, and being exposed to a variety of them really helped even things out. The fathers who took on roles in a Boy Scout troop tended to be exemplary anyway, so putting selection effects to work for us.
Maybe the postwar transition to single family housing is at fault? Less continuation of parenting knowledge when you no longer live in multigenerational homes.
That is also a factor, but I think a stronger impact has people less meeting each other offline. Even before internet, TV already had this effect -- the time you spend watching TV you don't interact with other humans.
You get an opportunity to see other people's parenting when you visit them at their homes. You lose this opportunity if each of you spends an evening by TV, or by a computer (even if you send messages to each other, you don't see their interaction with their kids); if everyone goes on a vacation alone (because that is easier to organize, and everyone has different preferences); etc.
Living in big cities also changes things. If you live in a small village, your friends are mostly in the same village, and it takes 5 minutes to see each other. If you live in a big city and your friends are 30 minutes away, you probably won't meet them just because you'd like to spend an hour outside. (My situation right now: I have small kids, my friends have small kids, it would be great to just drop them at the same playground and talk while observing them. But each of us has a playground next to them, which is easier than going 30 minutes by a car. If we lived in a village instead, the kids would naturally play at the same place.)
There is also a thing I observe in my country: before communism, there were many activities for people. During communism, everything spontaneous was illegal, but instead there were activities organized by the Communist Party. After communism, the activities organized by the Party stopped, but the original ones did not regenerate successfully. (Maybe because now they have to compete with TV and internet.) The few activities that exist for kids these days, are mostly organized by schools; which is good to have, but the problem is that the kids will meet there the same classmates they meet every day, instead of meeting strangers with a common interest, as used to be more frequent when I was a kid. For example, I used to attend a "math round" (voluntary extracurricular activity for math olympiad participants) when I was a kid; but as far as I know such thing simply does not exist here today.
I don't want to make too big generalization, because I may live in a bubble. But seems to me that, generally, people meet less offline. You need to be strategic about this; most people are not.