Why would this happen?

And yet, we're still at risk of nuking our only inhabitable planet over politics/religious/cultural differences.

Don't know where you got your figures. Here is just one of many that all agree yet contradicts your figures. And gives reasons for the increases:

Smithsonian Institution

http://humanorigins.si.edu/human-characteristics/brains

On the other hand jelly fish have been around for hundreds of millions of yrs with no brains whatsoever. Go figure...:-)

Something with quality and quantity :P

Is that true?

I can't tell.

Ego

Shrooms

are you suggesting we are starting to use more of our brainpower or something else?

because I think people are getting dumber