This sounds way too much like a cached thought to me. I'd like to see empirical data for that.

Note that privately held companies includes both companies held by a family (which tend to be less well managed because of the frictional costs associated with replacing upper management) and companies held by private equity firms (which tend to be well-managed). The NBER paper gwern linked through Hanson is available here, and if that link doesn't work for you I can email you a pdf.

A quote from it:

Table 1 also shows that private firms have higher profits, less cash, and more debt, even after we match on size and industry.