Financial markets are basically about information asymmetries, real and imagined, and financial regulation is largely about limiting those asymmetries to socially acceptable kinds and quantities. A general rule for trading success – perhaps the only useful rule for trading success – is: if you know something that nobody else knows and that will increase the value of a stock, then you should buy that stock! Afterwards, you should tell people. If you know something that will decrease the value of a stock, same thing, but with selling. If you don’t know anything that nobody else knows, index.
If you follow that rule too closely, though, you will end up in jail, as one does. So the trick is to know what kinds of secret information it’s okay for you to trade on, and what kinds it’s not okay for you to trade on. This is actually much harder than most people think it is,* which is why Doug Whitman is on trial.
My favorite category of nonpublic information that it’s maybe okay to trade on is your own intentions. If you wake up and say to yourself “I’m going to buy J.C. Penney stock today,” then right there you have an information advantage: you know something that no one else does.
Of course, who cares? In expectation, (1) you’re poor, so you’re not buying enough JCP stock to push up the price, and (2) you’re stupid, so your opinion of JCP won’t change anyone else’s view on the fair price. But occasionally you – not you you, but the “you” in this sentence – are rich and smart, and then your intentions are actually valuable information. One way you know that they’re valuable information is that stealing them is illegal: if Warren Buffett is secretly planning to buy Lubrizol, and his deputy knows about it, the deputy probably can’t go around buying Lubrizol. Another way to know: when Warren Buffett or Bill Ackman announces a position in a stock, the stock usually goes up.
So: is it okay for Bill Ackman to profit from his knowledge that he wants to buy J.C. Penney stock? Or does he need to tell everyone about his plans before he executes them, so that everyone can adjust their price in light of the knowledge that there’s a big buyer? Obviously you can’t arrest Ackman for insider trading because he traded on his knowledge that he was going to trade. (There are degenerate cases where you can come close.**) But you can argue about whether he owns that information and should therefore be able to profit from it, or whether instead that information should be public so he can’t take advantage of it at the expense of unwitting public investors who would have held out for a higher price if they’d known he was the buyer.