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.)
In each of these pairs, you have the same chance of winning in the second situation that you do in the first. This is garden-variety insider trading and, though I don't defend it, I do suggest that, although resentment-inducing, it simply introduces another unpredictable factor affecting the price of a stock.
If, however, insider trading is pervasive and institutionalized, the harm it causes is vastly greater, yet scarcely visible. It can destroy the financial system.
Before getting to this, what about supermarket lines and insider trading?
Consider the annoyance you feel when you're stuck in a supermarket line that doesn't seem to be moving. Other lines seem to be proceeding nicely past the cashiers, but not yours. You might even switch lines only to discover that your new line suddenly becomes as pokey as the one you left.
Unless you possess an unusually equable Buddhist temperament, this trivial delay can be really irritating and is one reason some stores have instituted single lines with the first in line going to the next free cashier.