I would agree to some extent, but I would also note that part of this "ineffective memory" is not necessarily a negative thing. In the
appendix to his rather controversial novel
State of Fear, Michael Crichton talks about Eugenics. Before WWII, lots of notable figures, including some of the greatest minds of the period, were proponents. "After World War II, nobody was a eugenicist, and nobody had ever been a eugenicist. Biographers of the celebrated and the powerful did not dwell on the attractions of this philosophy to their subjects, and sometimes did not mention it at all. Eugenics ceased to be a subject for college classrooms, although some argue that its ideas continue to have currency in disguised form."
Crichton isn't exactly accurate in the part about biographies. It wasn't just that certain notables had their records "expunged" (so to speak), as some (like Karl Pearson) are known by name by only because their work is still used. So even undergrads in just about any science have come across "Pearson's r", but despite the fact that Pearson was a founder of modern statistics, relatively little is ever written about him compared to other mathematicians from Newton to Cohen (the guy who finalized the answer to Cantor's continuum hypothesis). In math textbooks (among others), there are frequently little blurbs about the life of some mathematician whose work contributed to whatever section the blurb appears in. In the years I've spent teaching and tutoring mathematics (and most of my undergrad students have been social scientists who are taking a required intro stats course), I've lost track of the number of textbooks I've come across, yet I have rarely come across a blurb on Pearson. The one exception (I actually think it was in a work on the history of mathematics and/or logic) was really an explanation as to why so little is said of him: he was a eugenicist and was the Chair of Eugenics at the University of London.
The motivation behind this...shall we say, motivated forgetting? wasn't just embarrassment, but rather a concerted effort to expunge the
idea of eugenics from academia, which necessarily meant a fair amount glossing over certain people, certain works, etc.
Eugenics was a "science", with researchers, activists, and politicians all over the world supporting programs including research activity and legal/political action. The Holocaust was the purest, most effective, and most thorough realization of this "science". But it was so horrific that almost without exception it ended the political and academic support eugenics.
So while it is true that people are not regularly taught the history of academic and political eugenics programs/activities, this follows from a desire never to encourage that approach to evolution.
What mistakes? In the US, forced sterilization of those with "bad genes" were carried out, backed by state laws. The same was attempted in the UK, and although I don't believe it actually happened, there were similar acts passed by Parliment (on the "mentally deficient"). Canada too passed sterilization and seperation laws. The list goes on, and this is all in the 20th century before the Holocaust. After the Holocaust, the political, social, and academic support for Eugenics was replaced with a statement of basic rights regardless of race adopted by the UN. The departments, labs, textbooks, etc., which had all been devoted to "progress" using the "science" of eugenics, vanished. And recently (perhaps because our understanding of genes and evolution is sufficiently advanced, as it was only in 1953 that Watson & Crick's paper on DNA came out) the white-washing of history as far as Eugenics is concerned has steadily declined.
As far as I can see, the only "lesson" which we've forgotten is the other factor behind the Nazi regime. The extent to which Christianity is a cause is debated, because, while there is no doubt that wide-spread anti-semiticism in Europe and elsewhere was intricately tied to Christian views, the relationship between the Nazi's (including Hitler's personal beliefs) and Christianity is not clear. However, while this issue is debated all the time, the relationship between socialism and the Nazi's receives considerably less attention (at least in academia). It's fairly common for people to say that the Nazi party is an example of extreme right-wing views, while communism is one of extreme left-wing, but htis is rather fundamental distortion. What's interesting is that even in monographs, journal articles, and textbooks on totalitarianism which make this right/left characterization, the descriptions of the actual dynamics of regimes like Stalin's, Mao's, Mussolini's, etc., are quite similar to those of the Nazi party.
Since the French revolution, views usually associated with the Left have included a support communal government through a breakdown between the distinction of private citizen and government. Some realizations of this goal are supported even by some on the right-wing to an extent (social security, public education, etc.). And I'm not criticizing these, nor am I criticizing socialism as it is realized in Europe, Canada, and Australia.
However, Nazi germany (whatever rhetoric it employed against communism), was fundamentally a socialist enterprise: the People were the State, and the State the People. And in every instance of the extreme examples of this, a "messianic" type figurehead is there, whether it is Hitler or Saddam. Instead of religion (which represents by its very nature a rival to state/political power), an
social and
political ideology (as thorough and dogmatic as any religion) is promoted though propaganda and institutional action. Likewise, a figurehead arises to function as the symbolic representative of the State-sponsored ideology.
If there is any forgotten lesson, it is the relationship between the extremes of classical socialism (as it has developed both at an intellectual level and through political realization) and the Nazi party. Right-wing extremism is classically a monarchy (just look at Hobbes'
Leviathan), although more recently the right-wing has incorporated classical liberalism in its modern form (libertarianism), the extreme of which is anarchy.
What regimes like Hitler's, Stalin's, Saddam's, and others did is impossible without a great deal of state power and the capacity for the upper echelons to do what they will regardless of any and all views from the general public. Until recently (including, in some cases, very recently), we had Kings, Shahs, Emperors, and other monarchical governments capable of this. There was no equivalent of the Nazi party in the ancient world, or even the medieval. This is not true of the modern era.