As an outsider (most people in my country don't shoot guns for fun, but we still have our fair share of morons) I think not educating oneself/not being educated may be a more important cause.

My personal opinion is that it's more related to the way people spend (waste) their time. All of us, I mean. The way we (do not) educate ourselves, the way we do (not) value intelligence and knowledge.

• How many people shoot guns? vs How many don't ever read a book (a difficult one, I mean, say one essay a year)? or How many students reach university level without having read a single book? FFS, if that doesn't ring an alarm bell...
• How many people are (not) being taught how to have heated but articulated discussions, in the literal sense of debating against someone, having a dispute with someone, while still being able to not want to kill one another?
• How many people are willing to be told (and willing to admit that) they were wrong... when they were?

That lack of education and an overall cheerful ignorance of all facts that dare not fit their viewpoint, no matter which one it is, seems to me a much more likely cause to explain why more and more people around the world (not just Americans) 'seem cognitively impaired'. And that's because, well, they are. Sadly.

We don't value knowledge anymore, we value money and success. Once again, suffice to ask people: how many essays did you read in the last 12 months? Or to look at kids, how many of them want to be, say, a doctor, a scientist of some sort or, even funnier, a writer? And how many want to become 'an influencer' on YT (or TikTok, or whatever) or to become some star singer or sport star?

Kids have not suddenly become allergic to smartness. They're only the mirror of what our real values as a society are (not the ones we pretend to have). Which are not being smart, not even talented as a matter of fact. They are: easy money and success.

imho, this is the main cause of dumbification going on everywhere. Obviously, I may be wrong and maybe I should stop eating lead bars as a snack?

Religion is the cause. You are not allowed to think outside the cult. There are a lot of idiot Americans who don't own guns or are exposed to them, so the lead theory is not valid in that sense.

what could explain the incredibly high numbers of Americans who seem incapable of critical thinking

Garbage education system.

It's really the idiocracy theory. Dumb people have more and more kids while smart people tend to have 0 to 2 kids. It's exponetially growing the amount of dumb people. Besides some people that had potential dumbed themselfs down by joining organized religion. very sad

I figured copper jackets would greatly reduce lead exposure, which is all I used when I used to shoot.

You’d have to shoot thousands of rounds in an unventilated room to get even close to one day of leaded gasoline exposure

Leaded gas wasn't fully stopped until 1996. Still in some aviation used (piston plane engines).

But yes I wonder about shooting ranges too. I think a couple times a year at an indoor range isn't insignificant.