As quantitative investors and other asset managers try to deliver private equity-like returns through indexes and stock funds, a new study from the University of Pennsylvania’s Wharton School suggests that it is not so easy to do. This according to a recent article in Institutional Investor.

“Equity investors are increasingly seeking to deliver private-equity-like returns in the public markets, but it may not be possible to replicate some of the drivers of leveraged buyout performance outside of private equity,” the study found, concluding that a large portion of private equity returns are attributable to different behavior patterns by those firms.

“In particular,” the article explains, the study reports that private equity firms “act as deep pockets investors: when the internal cash flow of their portfolio companies is insufficient to fully finance investments and/or make required debt payments, the private equity owner steps in an provides additional capital.”

The study, which analyzed empirical data on 407 buyouts in the U.K. (where private companies are required to file annual reports) concluded that the “abnormal returns of PE firms cannot be replicated by other investors.”