>On the other hand, if one accepts that this is not due to any inherent racial trait, then one has to look at American culture and history with an open mind and try to understand how it came to be that way

Absolutely. My disagreement comes from the presented false dichotomy of the explanation being either racial inferiority - which neither of us believe - or ongoing systemic discrimination which (in my view) can only explain some of the problem. Further, I think that over-reliance on it as an explanation blinds its advocates to other possible causes and therefore other solutions.

Let's take a toy example for simplicity; a game of Monopoly. Systemic economic discrimination would take the form, perhaps, of one player having favorable rules; they get to reroll their dice, or get more money from passing Go!. Clearly unfair, and the solution would be obvious; equalize the rules. However, there are other ways the game can be tilted. If one player starts with more money, i.e. benefits from historical wealth, then they are likely to do win more often. This historical economic oppression is not the same as systemic discrimination - the rules affecting each player can be identical once the game actually begins - and requires different interventions to resolve.

I recall reading some time ago that immigrants to America from African nations tend to do better than the background African-American population, despite often coming with even less than a typical African-American family would have. One would expect them to experience just as much systemic discrimination, and they are just as impoverished as those who have experienced historical economic oppression, so that also does not seem to explain things fully. We can then turn to matters of culture and environment. If one of the players in the hypothetical Monopoly game pursues a non-optimal strategy (by refusing to buy real estate[0] for example) then they are likely to lose even with equal rules and starting assets.

The cultural explanation also helps understand why the Japanese, literally interned as prisoners en masse during World War II, and the Chinese, imported in their thousands as low-wage labor to build the railroads and both banned from owning property until 1952 [1], were able to bounce back into two of the most wealthy population segments in the country. Strong social cohesion and an understanding of long-term beneficial life strategies both help oppressed minorities overcome their situations. This is entirely independent of discrimination, which is why it is so frustrating to see, for example, no less a source than the National Museum of African American History and Culture [2] label traits like "objective thinking," "hard work being the key to success," and "delayed gratification" as aspects of white culture, and therefore alien.

Let's take another example - crime statistics. The "classic" systemic discrimination explanation of higher numbers of black people being incarcerated denies that actual crimes committed are higher and statistics showing that they are is a symptom of racist police forces and justice systems. To solve it, we need police and prison reform, racial sensitivity training, etc. The historical economic oppression explanation instead concedes that yes, black communities tend to have higher crime, but that is because of fewer opportunities available and recommends investment into education, reform of certification and some financial laws to make it easier for small black businesses to get started, and stronger investment into infrastructure to make it easier and safer for the population of inner city communities to access the job market. The cultural explanation also concedes that there is more crime there, and that black people have the power to solve that amongst themselves, by cooperating and developing stronger social norms that discourage violence. In one remarkable example, such an effort gave birth to hip-hop culture [3]!

Each explanation points at part of the problem; I hope you'll agree now that labelling only one as correct and sufficient, and saying that everyone who disagrees with it can only explain the problem through racism is non-productive. Even if the cultural and historical explanations are rooted in systemic discrimination of the past, they demand different solutions in the present to resolve, just as making a playing field even after one player has already accrued a significan advantage would not lead to a fair game.

[0]: https://www.youtube.com/watch?v=GlKL_EpnSp8 (NSFW, explicit visuals and lyrics)
[1]: https://en.wikipedia.org/wiki/California_Alien_Land_Law_of_1...
[2]: https://archive.is/gyYNs
[3]: https://www.bloomberg.com/news/articles/2015-10-23/disruptin...