Just to add the point of the argument , it is not that "white people cannot experience downward mobility". The point is what explains this statistical difference, when making a statistical model different variables get accounted for. By comparing people born with the same socioeconomic status you are in effect controlling for this confounding factor. Thus the scientist are making the point that prejudice has a role in "who gets a seat at the table" on a population level. Sorry if that was not made clear in the text . Individuals can always be exceptions . For an analogy to make it easy to understand the average height of basketball players is 6.7m yet there are exceptions like Muggsey Bogues. On a whole we can generalize that being "tall" for instance is a advantage in a sport like basketball even though it is not impossible to be short and in the NBA. Similarly I am arguing that being white in North America confers an advantage due to some people being prejudiced in our current system.