什么原因导致黑人大学毕业生失业率高于白人大学毕业生

昨天一个NPR Host 在谈论 the obstacles facing people of color 时， 说了个统计数据：

During the recent recession, black college-educated men had twice the unemployment rate as white college-educated men. The unemployment rate at the height of the recession for white college-educated men was 4 percent. For black men it was 8 percent. ...

这是由于discrimination 还是affirmative action 用在大学的坏处？  我觉得她（an African American lady) hinted the former ...