What do mutual fund managers' private portfolios tell us about their skills?

I study a registry-based dataset of Swedish mutual fund managers' personal portfolios. The majority of managers do not invest personal wealth into the very same funds they professionally manage. The managers who do invest personal money into their funds subsequently outperform the managers who do not. The results suggest that fund managers, in contrast to regular investors, are certain about their ability to generate an abnormal return, or lack thereof, and invest their personal wealth accordingly.