http://www.nytimes.com/2011/08/20/us/20school.html
Review of Census Data Reveals Information Tied to Schools
By SABRINA TAVERNISE
New York Times
August 19, 2011

...

But degrees mean higher earnings. The average annual pay for a worker with a bachelor�s degree was $58,613 in 2008, nearly double the $31,283 earned by workers with a high school diploma only, the bureau said.

<end of excerpt>

The first sentence above implies that the degree causes the entire differential, but I think it only explains PART of the differential.

"To see what is in front of one's nose needs a constant struggle." - George Orwell