Inbred Economist Professors?

A day after I noted the possible inbreeding of law professors, Tyler Cowen posted this, excerpts from an interview:

He [Cowen] said that he agreed with the idea that influence of economics comes from a relatively small number of institutions, and he thinks the number is shrinking. “What used to be something like a ‘top six’ has over time become the ‘top two,’ namely Harvard and MIT.”

I've not checked to see how many of the Harvard/MIT economics professors graduated from those universities--I suspect a lower proportion than with women law professors at Harvard, but it looks as if they're on the same path.  It's a logical extension of trends: education is important, so the best education is very important, so it's best to hire only those with the best education.