Insider Trading: Supermarket Lines, the Stock Market and Chance
Columnist compares superfast trades to the supermarket fast lane.
Dec. 6, 2009 — -- Before getting to the topical issue, let me start with a simple puzzle.
Which of the following two situations would you prefer to be in?
In the first one, you're given a fair coin to flip and are told that you will receive $1,000 if it lands heads and lose $1,000 if it lands tails.
In the second, you're given a very biased coin to flip and must decide whether to bet on heads or tails. If it lands the way you predict, you win $1,000 and, if not, you lose $1,000.
Although most people prefer to flip the fair coin, your chances of winning are 1/2 in both situations, since you're as likely to pick the biased coin's good side as its bad side.
Consider now a similar pair of situations.
In the first one you are told you must pick a ball at random from an urn containing 10 green balls and 10 red balls. If you pick a green one, you win $1,000, whereas if you pick a red one, you lose $1,000.
In the second, someone you thoroughly distrust places an indeterminate number of green and red balls in the urn.
You must decide whether to bet on green or red and then choose a ball at random. If you choose the color you bet on, you win $1,000 and, if not, you lose $1,000. Again, your chances of winning are 1/2 in both situations.
Finally, consider a third pair of similar situations.
In the first one, you buy a stock that is being sold in a perfectly efficient market and your earnings are $1,000 if it rises the next day and negative $1,000 if it falls. (Assume that in the short run it moves up with probability 1/2 and down with the same probability.)
In the second situation, there is insider trading and stock manipulation -- a company official confidentially tells his niece who tells her botox doctor who tells all his patients -- and the stock very likely rises or falls the next day as a result.
You must decide whether to buy or sell the stock. If you guess correctly, your earnings are $1,000, but, if not, you lose $1,000. Once again your chances of winning are 1/2 in both situations. (They may even be slightly higher in the second situation since you might be one of the doctor's patients.)
