The authors suggested that unexplained factors could include discrimination, differences in school quality, and differences in opportunity.

It's hard to argue that there's more discrimination these days than in the 1970s. Conversely, in the 1970s, after the passage of the Civil Rights Acts, the country was gung-ho about actively minimizing the effects of discrimination. That effort has almost certainly decreased to the pre-Civil Rights Era (despite what parental rights inquisitors would have you believe).