Via Wharton’s newsletter:

It’s always been assumed that when employees leave their companies to join other ones that all their knowledge and experience leave with them. But new research suggests that, at least in the high-tech field, firms can wind up gaining access to the knowledge being generated at their former colleague’s new place. The results of this research are presented in a paper titled, “Learning from Those Who Left: The Reverse Transfer of Knowledge through Mobility Ties,” by Wharton management professor Lori Rosenkopf and Wharton doctoral student Rafael A. Corredoira.

…

“Contrary to the view that companies lose something when a worker leaves, the study found that they stood to gain. Specifically, firms that lost an employee to another firm were 8% more likely to cite that firm than other equivalent firms, Rosenkopf says. The reverse flow of knowledge was particularly pronounced when the employee moved to another region. Then the old firm was 22% more likely to cite the new firm.”

http://knowledge.wharton.upenn.edu/article/1565.cfm