An unpopular opinion, but circa 1970 and many of the effects in these charts likely represents an inflection point where the US population at large left its Christian heritage.
I have a bit of a hard time tracking how, say, removing the Ten Commandments from schoolrooms changed the ratio of gains between the 1% and everybody else. Or church attendance.
But other people have made a case here that more women in the workforce was a plausible factor. They point to birth control as a key to that. But I suspect that abortion also contributed, and it was legalized in 1972.
That was just one small part of a society-wide change of worldview. Prior to 1970, Christian values, ideals, and beliefs were assumed by the majority of the US population and respected by all but a tiny minority.
However, even by 1900 the philosophical basis for this shift had been laid, starting in 19th-century France and Germany with enlightenment (and particularly existentialist) philosophy, like that of Nietzsche and Sartre, joined by Marxist atheism, and finally making its way to the US and the rest of the post-WWI Western world as secular humanism--and to weaker parts of the Western world, and other places as well, as full-blown Socialism/Communism.
