I think cultural Differences is the most responsible of the gap TODAY.
Does not mean that other factors are not in play.I certainly do believe that a solid 5% of dis-proportionality in success is due to racism... and you can make an argument that the negative impacting cultural differences that have arisen is a result of past racism and history... and I would probably agree 100%... still doesn't take away from the fact it is the culture that is the problem.
I would give it a 10% of difficulty of class mobility... but i think that is largely a product of culture, there are ethnic groups in the united states with cultures that have had a high rate of social mobility.
I think there is an argument for genetics being a factor. (And I am including a disclosure: Just because there seems to be differences is averages in ANY traits height, weight, speed, IQ, disease vulnerability, food allergies, etc. between groups of humans, like classifications such as race...does not mean that this fact should be an excuse to discriminate, because these traits are largely bell curves with a lot of overlap, and are no means to judge an individual based on race; that would be extremely discriminatory and unethical). Though how much of a factor is largely unknown and highly controversial. Inheritance of IQ, according to scientists, is around 60% nature 40% nurture... though even this is argued to being more like 40/60 instead of 60/40.... regardless, IQ, which a attribute which has the highest correlation with success in America. And African Americans tend to have around a full standard deviation below the mean in average IQ. WHY? we don't entirely know, a significant amount of people believe it's because of bias in IQ tests, or that it's largely because of the Nurture and conditions the average black family is in... though studies have shown IQ is significantly inheritable.
And to those that think IQ is bogus...well, like I said before, there is no other trait that is more highly correlated with success, and these tests have been refined in the top universities for a hundred years to reduce as much bias as possible even testing children and extremely young ages.... so it's certainly measuring something. The second highest trait that is correlated with success is Conscientiousness... which thankfully can be a learned behavior that anyone can have.... and that means anyone can be successful!!!
So if I were to put it all together on what my opinion is, I would say... the reason why the african american population has a statistically lower rates of success.
80-85% Culture(Like I said before, a culture that has been bred out of racism... and in a way was caused by it)
5% racism
10% Difficulty of social mobility
0-5% Genetics