Over the past two quarters, the S&P 500 lost 19.23%. As horrific as that has been, that actually ranks only 78th out of the 2,409 six-month periods since 1800. Believe it or not, 77 six-month periods have been worse.

In eight six-month periods, the S&P 500 lost more than 40%. In 18 periods, it lost more than 30%. The very worst single six-month period was the one that ended in May 1932, in which the S&P 500 lost an astonishing 52.92%.

Of the 77 six-month periods worse than the most recent one, 21 were before 1900, and none of them occurred during the Civil War. Thirty-three (including the very worst four) were in the 1930s, in the Great Depression that followed the stock-market crash of 1929.

The worst six-month period in the modern era ranks 13th overall. It was the one that ended in September 1974, with the S&P down 32.39%. Four others were in the 1970s.

The most recent period worse than the past six months ranks 73rd. It was the one that ended in March 1988, with the S&P down 19.56%. That means that in the 1990s, no six-month period was worse than the most recent one.

Good and Bad News

This shows that what we've experienced over the past six months is by no means as bad as it could have been -- or could still be. But at the same time, it shows that market performance this bad is extremely rare. The S&P 500 has done this badly (or worse) only 3.2% of the time over a span of 200 years. But don't let any of that make you think it can't get worse.

It can. Of the 77 six-month periods worse than the most recent, the S&P went down further during the next six months 24 times -- losing an average of an additional 18.24%.

OK, so the bad news is that, based on history, we've got about a one in three chance of things still getting worse over the next six months. But of course, the good news is that we've got, on the same basis, a two in three chance of things getting better.

But not that much better. Over the 53 times the S&P rose over the next six months, it rose an average of only 18.38%. Even if that happens now over the coming six months, that's not going to make up for the 19.23% loss of the past six months. And don't forget about the bad magic of compounding losses. If you start with a dollar and you lose 19.23% of it, you've got 80.8 cents left. It that then rises by 18.38%, you're only back up to 95.6 cents.