http://www.forbes.com/sites/jacquelynsmith/2013/03/08/the-pr...

I think this is the actual study?

http://www.nerisearch.com/neri-newsletter-career-development...

Basically, a skilled employee gets the most out of a job in the first 3 years. And the new company gets the most out of the employee in the first 3 years (or so). After that, in order to restart the learning experience for both parties, a "change of venue" is necessary.