Imagine your doctor tells you that you have an unhealthy condition. You ask what you can do about it. He responds that he doesn’t know. When you return some time later, he tells you your unhealthy condition is improving. You ask what brought on the improvement, and he responds that he doesn’t know. You ask if any studies are going on about the condition. He shrugs and says a few.
High school graduation in the U.S. is like this. First, researchers struggled to understand why it was low and unchanging for a long time.