It happened almost every earnings season. Our hedge fund would own a million shares in some company and two weeks before it was to report quarterly earnings, its stock would start dropping. There was no news to explain it. We were in the dark, even though it was my job to know. Inevitably, the company would report a disappointing quarter, missing Wall Street's earnings expectations by a penny or two. Someone knew. A salesman's brother-in-law heard a few deals didn't close. Or maybe an insider was singing.
The recent arrest of Galleon Group hedge fund's Raj Rajaratnam on insider trading charges puts a spotlight on this game. Is trading on industry knowledge widespread? Absolutely. That's how many hedge funds and mutual funds get an edge. Is insider trading also widespread? Only the Securities and Exchange Committee's wire-tappers know for sure.
It's a short walk from running an information network to being an insider.
Stock markets trade on information. Millions of people generate billions of trades every day. Each trade contains a tiny piece of information built into it. ("I think Apple is killing Nokia" or "I think GM is toast.") Eventually we are proved right or wrong, and we make money or we don't. In the long run, the market is always right. On any given day, your guess is as good as mine.