[lumost]

Alternately, big brains are recessive/selected against in all circumstances except those from 6mya to 70kya. We've generally removed most selective pressure at this point, if having big brains is due to a combination of recessive traits we'd expect brain size to start shrinking.

[snek_case]

I was going to say: maybe it's because we're so successful? We dominate our environment so well that we've made the world a much less threatening place, and so it's not as difficult to survive in the world anymore. Less selective pressure means we end up with a less selected population.

I don't want to turn this into a political commentary, but the modern age is an extreme example of this. You could be the kind of person who doesn't finish high school and can't keep a job, a deadbeat, but still manage to have multiple kids. Even if you don't take care of them, someone else will ensure that your kids survive. Long ago, not being able to are and provide for your kids probably meant that they died.

IMO, as we keep making the world increasingly more safe and comfortable, we might have to resort to genetic engineering. Otherwise, it might be inevitable that more and more genetic disorders will arise, just because we're able to keep people alive with all kinds of illnesses that would have been fatal before. People can argue about the ethics all they want, but it seems inevitable.

[SamoyedFurFluff]

Wouldn’t being able to retain maximal genetic diversity actually be a good thing species wide though? Because it also means we have ever bigger achievers in ever more niche but valuable roles.

[snek_case]

We can probably keep 99% of the diversity and just filter out diseases that significantly decrease quality of life.

[im3w1l]

Screening and abortion are already used to prevent serious disorders. For instance, a huge fraction of downies are aborted. But subtler things are more challenging.