Extremism is born from the real sleeping danger of poor education and information standards which allow people to latch onto and be swayed by simplistic nonsense touted all too loudly on social media. When the rest of the world wore masks to protect the person next to them a loud section of the US population believed it was a plot to turn them Muslim. Similarly when the world understood the value of the developed vaccines a large part of the US thought it was a plot to implant tracking microchips - while carrying phones that were actually capable of tracking their activity. Every country has factions of this sort and the American population might have more reason than most to suspect government instructed mass vaccination programs but for a country with so much wealth the USA seems particularly prone to the sort of demagoguery that can take root in a poorly informed population. To put it more succinctly every day the film Idiocracy gets a little less funny.
Although most of what I said put forward the idea that the primary cause was indeed that the education system is not fit for purpose and in a wider sense post-school age education in the form of dissemination of reliable information to the population is similarly flawed, it is also somewhat true in trivial reference to Idiocracy that eugenics do play a minor additional role in that the middle classes are finding it increasingly difficult to find both time and the finances to raise children. The second point you made is vaguely contrary to the ideas of organic evolution, although admittedly intelligence is only one of many selective traits to the point where as you say any attribute of the parent is in no way guaranteed to be passed to the child.