They are not really exctinct, they are part of our ancestry, at least in most of Europe and those who migrated from Europe. One possibility is that when Sapiens met Neanderthal they simply outnumbered them because they came from more fertile/hospitable lands. Like Google acquiring an older but smaller company.

I'm 2.3% Neanderthal.

We're not extinct. We merged.

Another aspect, is theres no telling how much that 2.3% matters. Not all strands of DNA have the same effect on how someone turns out

2.3% doesn't reflect what i'd call a merge..

So do you only merge your code when you've edited more then 50% of your lines?

Unless neanderthals were 2.3% of the population at the time they merged.

or, the populations were 50/50 and the humans were like, "let's bone the hottest 95.6% of neanderthals and kill the rest"

Other way round

I originally wrote the joke the other way round. When I switched the order I forgot to change the number back.

That's all for today's episode of Inside the Joke

Who approved the PR?

Off the top of my head, disease, in-fighting, or some other missing adaptation that made them less able to flourish and reproduce in as-high numbers.

Over specialization perhaps. High calorie diet and starvation? Harder to cope in a warming environment?

larger brain does not always mean smarter.

I believe that all evidence points says that it helps.

even people with hydrocephaly? Neanderthals , although similar to modern homo sapiens, are not the same.

I don't think it's useful to consider a rare (0.2%) neurological disorder in this discussion, unless you're suggesting that Neanderthals had voids in their brain matter. This would be an extraordinary claim, being unlike any other primate or mammal.

For humans, bigger is almost always better.