In a perfect world, an ample supply of a commodity at a low price should stimulate demand. Not so with gasoline in America. According to MarketWatch, "Even though prices at the pump are now about 45% lower than they were a year ago and significantly below $2 a gallon, 52% of Americans told Gallup that they have not gone back to their old gas-guzzling ways."

The poll indicates, as might be expected, that lower income drivers are cutting back the most.

The easy reasoning behind the drop in driving is that people will save money in a tough economy. If things were good, drivers would drive more and take advantage of falling prices.

It may not be that simple. The average American is not a boob. He understands that, over time, the amount of oil in the world is finite. Gas prices may not go back up tomorrow, but they will go back up. Getting used to using oil-based products less is good long-term thinking.

Another, less obvious, reason for cutting back on hours on the road is that a car driven less lasts longer. Buying gas may be getting less expensive, but a new car is still an investment of over $20,000 in most cases. People don't have that money. Getting another year of service out of an aging vehicle has a particularly important value in a recession.

Douglas A. McIntyre is an editor at 247wallst.com.

The Dow Jones industrial average soared today -- up 183 points on a raft of merger news, strong earnings from drugmakers and more confidence in general about the strength of the economy in the face of higher interest rates. (Or maybe it was Barron's cover story, as some fellow bloggers believe).

But those aren't the only reasons stocks perked up. Investor pessimism probably also played a role.

"huh?" you say.

Well, many professional investors have long believed that investor sentiment is a reverse indicator of stock market performance. In other words, if investors are feeling optimistic, that's a sign stocks are about to turn lower (after all, anyone who could or would buy stock probably already has). Conversely, if investors are pessimistic, that means stocks could well head higher. There is probably a lot of money on the sidelines waiting to jump back in on any signs of good news, the logic goes. (Bernie Schaeffer has built his research firm on this idea).

According to a new Gallup Poll just out today, investors are now the most pessimistic they've been all year. The last time they were this gloomy was November 2005. The optimism index is down to 55, from a recent high of 55 in May of 2006. Investors are apparently more worried about the overall economy than their personal situations. Their chief concerns are rising energy prices, the Federal budget deficit, Iraq and outsourcing of jobs (in that order).

Usually the relationship between investor pessimism and rising stocks doesn't play out on the exact same day in such a clear fashion. Nonetheless, the investor sentiment theory is good to keep in mind when you're doing your own pre-trade gut check.

Feeling the urge to buy some stock? Maybe it's a sign the market is topping out. Feeling the urge to sell? Maybe you should be buying instead.