At the very top departments, more than 90 percent come from the worldwide top-35 departments; the top is almost entirely self-regenerating.
They call the result "groupthink," despite some misgivings about the term. I think they should listen to their misgivings and change the term. I agree with them about what causes the ideological concentration. I wrote,
There are more Ph. D's produced each year at top schools than there are job openings at top schools. On average, then, students get placed in lower-ranked economics departments than the ones in which they are trained...
Suppose that you think of yourself as the "child" of the professor who signed your dissertation. Thus, I am Solow's "child." But I have not given Solow any grandchildren.
In order to have lots of grandchildren, you need to have lots of highly productive children. In order to have lots of highly productive children, it is necessary--and far from sufficient--to teach where there are lots of top-flight graduate students. Basically, if you are teaching anywhere other than Harvard, MIT, or Chicago, your chances of having lots of grandchildren are pretty low.
In some sense, most professors in any discipline are "descended from" just a handful of senior eminences. Unless those eminences happen to hold diverse ideological viewpoints, ideology will become highly concentrated.
The problem is not one of social psychology. It is institutional/ecological. Academia is set up so that within each field a small group out-breeds everyone else. Economics happens to have a relatively diverse ecosystem, but it's all thanks to the University of Chicago's eminence. If Milton Friedman had been a conventional left-winger, what would have kept economics from turning out like sociology?