buy high?

Today's stock market exhibits plenty of worrisome signs. Stocks of small companies have been sinking, which is often a bad sign for the broader market. Economic growth remains sluggish years after the worst recession since the Great Depression. Vladimir Putin's mischief ultimately threatens the crucial flow of natural gas from Russia to Western Europe.

Why buy now, when the leading market indexes are at record highs?

Because a review of recent history shows that the date you pick to invest doesn't matter that much, even if you invest at the market's highest level of the year.

Sound crazy? Dan Wiener, editor of "The Independent Adviser for Vanguard Investors," compared the records of two hypothetical investors over the past 30 years. Each started by investing $1,000 in the Standard & Poor's 500 index ($INX +0.46%) at the end of 1983. (Of course, you can't buy an index, but you can invest in a low-cost index fund.)

Over the subsequent 30 years, each investor put $1,000 annually into the S&P 500. But investor No. 1 bought on the last trading day of the year, while investor No. 2 bought at the S&P's highest point each year. In other words, investor No. 2 bought on the worst possible day each year.

Here's the surprise: At the end of 30 years, investor No. 1 had achieved an annualized return of 9.9 percent, while investor No. 2 earned an annualized return of 9.5 percent. The difference in returns was just 0.4 percentage point per year, on average.

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Of course, if you could accurately time the stock market, you could enrich yourself enormously. Over the past five years through April 30, the S&P 500 returned a sizzling 19.1 percent annualized. But from December 31, 1999, through April 30, the index returned only 3.7 percent annualized. So clever market timing would have done far better than buying and holding through this period, which included two vicious bear markets.