Since the start of this century, The Times found, price fluctuations of 4 percent or more during intraday sessions have occurred nearly six times more than they did on average in the four decades leading up to 2000. The price swings today may feel even more notable because the 1990s represented a relatively calm time for trading. In contrast, price fluctuations of 1 percent and more during intraday trading were more common in the 1970s and 1980s.

As for closing prices, the more-frequent jumps could also be clearly spotted. Thirty percent of trading days since the start of 2010 were up or down more than 1 percent at the time of the closing bell. That's far more than the 20 percent of such jumps in the 1990s. The trend toward greater volatility is more pronounced in larger price moves.

Regulators at the Securities and Exchange Commission have been looking at changes in the markets and automated trading strategies in connection with volatility. The market is no longer based on one single exchange but is fractured across four big exchanges and several smaller forums. High-frequency traders, using powerful computers to trade at exceptionally high speeds, now account for up to 60 percent of daily turnover.

And in the last decade, exchange-traded funds have become a large factor in trading, after hundreds of billions of dollars were poured into them. Those funds - like mutual funds, but traded daily - tie their values to indexes or bundles of stocks rather than individual companies.

But even as financial problems simmer in America and abroad, officials have yet to pinpoint exactly why stocks seem to move more quickly and to greater extremes.

Some financial historians question whether the markets are in a "new normal" of permanently heightened volatility.

"The last few years have been the most volatile for all of recorded history," said Andrew Lo, professor of finance at the M.I.T. Sloan School of Management. For evidence, he says that 10 of the biggest 20 daily upswings and 11 of the largest 20 daily drops since the beginning of 1980 to the end of last month have occurred in just the last three years.

David Fuller's view Some quoted in this article attribute the increased volatility to uncertainty. Well, there has always been uncertainty, leading to disagreement, causing some stocks and other instruments to move higher while others moved lower. The big difference this time is that they move in the same direction, witrh few exceptions, until the daily trend suddenly reverses.

I attribute it to high-frequency trading (HFT) which barely existed before the turn of the century and now accounts for most of the daily volume on Wall Street (see also Email of the day 4 below).