I think this result is obvious to anyone who has spent any time in the academic world, although it is nice to see some solid numbers behind it.
The harsh truth is that key to academic career advancement is who you know much more than what you know. I every single person I knew in graduate school who got a postdoc position did so through informal means (i.e. knowing someone who knew someone), and having letters of recommendation written by the right people from the right departments at the right schools opens all sorts of doors to the academic hierarchy that would otherwise be closed.
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