Stocks rang up their best September performance since 1939, with the SPDR S&P 500 ETF (NYSE: SPY) gaining 9%, and added to those gains this month. In a market still marred by economic fragility and huge uncertainty, who's buying stocks right now, and are the resulting gains fleeting or durable?

In one of his daily commentaries in mid-October, David Rosenberg suggested that the buying originates from three areas: pension funds, bank proprietary trading desks, and hedge funds. I like Rosenberg for his provocative, contrarian commentary, but I think he got things wrong here. Of the three parties he fingered, two look like highly unlikely suspects, with just one of the three a natural buyer of stocks in this market.

Pension funds: Unlikely
Rosenberg cites an Oct. 13 Wall Street Journal article regarding the enormous unfunded liabilities at municipal pension funds. Rosenberg assumes that because pension funds' future return projections remain unrealistically high, they feel compelled to reach for returns by moving further out on the risk curve into equities.

If that is what pension funds are now doing, it would be contrary to the way they have behaved in the aftermath of previous stock market crashes, when the sting of losses created a bias against buying stocks. Sure enough, the same Wall Street Journal ran a front-page article three days later entitled "Pension Funds Flee Stocks in Search of Less-Risky Bets."

Proprietary trading desks: Unlikely
Here, Rosenberg cites Fed data on aggregate bank trading assets, "which have soared $50 billion alone in the past month." Unfortunately, it appears that he was looking at an account that specifically excludes most securities held in trading accounts. In fact, the Fed's data aggregates the bulk of trading assets with "held-to-maturity" and "available-for-sale" securities, so it's impossible to tell whether trading assets have increased. As far as I can see, the only sub-account that contains equities is roughly flat over the same period.

More generally, it's hard to believe that banks' proprietary trading assets have "soared" even as some of the largest bank proprietary trading desks have been shuttered in response to the so-called "Volcker rule," which establishes limits on banks' own-account activities. Last week, Goldman Sachs' (NYSE: GS) CFO confirmed that the firm has largely unwound the U.S. arm of its storied proprietary trading desk. (Goldman was used to generating 10% of its revenue from proprietary trading and private equity investments.)

Hedge funds: Probable
With hedge funds, I think Rosenberg is onto something. The HRFI Equity Index has the long-short equity funds gaining just 4.5% for the year through Sept. 30, and 3.5% for the month of September -- both of which lag the stock market's performance over the same periods. With the end of the year fast approaching, it wouldn't be surprising to find that equity hedge funds are willing to increase their exposure to stocks in the last quarter. The large flows into stock-related exchange-traded funds in September also support the notion of increased hedge fund participation. (ETFs are very popular with hedge funds.)

Here are three stocks in which hedge fund ownership has recently and dramatically increased:

Wendy's/Arby's Group (NYSE: WEN)
Total hedge fund ownership of Wendy's/ Arby's now exceeds 30%, with Nelson Peltz's hedge fund Trian Fund Management owning 22%. In June, Peltz disclosed that a third party had approached Trian about the possibility of a takeover of the restaurant company, the nation's No. 3 fast-food chain. Morningstar believes that Wendy's/Arby's is a potential LBO target.

J.C. Penney (NYSE: JCP) 
At the beginning of the month, Pershing Square Capital Management, the hedge fund of activist investor Bill Ackman, disclosed that it had taken a 16.5% and a 11% stake in J.C. Penney and Fortune Brands (NYSE: FO), respectively. Pershing Square thus became the largest shareholder in both companies. At J.C. Penney, expect Ackman to promote a financial engineering plan to boost shareholder value. The plan will almost certainly turn on J.C. Penney's real estate holdings -- a formula that Ackman has applied in the past with varying degrees of success. At the end of January, J.C. Penney owned 416 of its 1,108 stores.

Fortune Brands
Fortune Brands looks like an interesting target for an activist. With its ragtag collection of well-known brands spanning different sectors (from Jim Beam whiskey to Master Lock padlocks to Titleist golf clubs), it's quite possible that the company suffers from negative synergies, where the whole is worth less than the sum of the parts. Perhaps Ackman will lobby for a breakup of the company along the lines of its three main activities (premium spirits, home and security, and golf) to unlock hidden fortune.

Long-term upside, short-term volatility
If hedge funds are driving this market higher, the gains could well be fickle. This need not concern long-term-oriented investors who are focused on purchasing undervalued stocks; still, it's worth being aware of the possibility, since it could lay the groundwork for increased volatility over the next several months.

David Gardner recommended Fortune Brands in 2009, producing a 43% gain (so far!) for Stock Advisor subscribers. Discover more of our powerful picks in our new free report, "5 Stocks The Motley Fool Owns – And You Should Too."

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