Odds are good that the US stock market will be higher at the end of December than it is today. Don’t get too excited — these odds have nothing to do with how strong the US economy is right now or the stock market’s impressive returns so far this year. In fact, the chance of a winning second half of 2021 would be the same even if stocks were in a bear market or the economy was in recession.

The reason why has to do with the stock market’s efficiency. Its level at any given point reflects all available information up to that point. For example, if on 30 June the odds were better than usual that stocks would be higher in six months’ time, traders would have already bid up prices to take those better odds into account. They wouldn’t wait until later in the year to up their bets.