Look at a graph of the Dow Jones Industrial Average (DJIA) over the last 10 years or so, and you’ll see an odd symmetry – today’s levels are  about the same as they were in the 2000-2001 era. Adjust for inflation and the difference is even starker: The DJIA would have to be around 15,000-16,000 today simply to be equivalent to the DJIA level of about 12,000 a decade ago.

For many years it seemed a truism: Stocks outperform other investments “in the long run.” Buy and hold through thick or thin was the watchword — the ticket to eventual financial security. But can you still take that to the bank?

A recent Wall Street Journal story notes that big changes are underway in financial markets. “Investors are abandoning the time-tested ‘stocks for the long run’ optimism that dominated since the late 1980s. Instead, there is a widening belief that the mess left behind by the housing bubble and financial crisis will be a morass to contend with for years.” The article also noted that developed market equities saw wholesale withdrawals of more than $100 billion since June, and that the new consensus sees equity returns in the foreseeable future below the long-term average of 9% to 10%, with prices lingering “at below-average valuations. Conservative strategies aimed at collecting stock dividends, out of favor for decades, are coming back in vogue.”

But Jeremy Siegel, a professor of finance at Wharton, says that relative to other investments, today’s stock market values still present a buying opportunity. “In bear markets, after long periods of poor performance, everyone questions the long-run success of the stock market. This is not surprising at all. Just as a big bull market brings in a more optimistic estimate of how high we will go, bear markets bring out more estimates that markets are in for a poor time over a number of years.”

This has been a tough decade for stocks, but Siegel has pointed out in the past that over the last 30 years, even taking into account the last decade, stocks have outperformed most other alternatives. Knowledge@Wharton asked Siegel a few additional questions on the topic this week.

Knowledge@Wharton: There has been a lot of money withdrawing from equity markets. What do you make of the trend?

Jeremy Siegel: Yes, people are pulling out of developed market stock markets — they pulled out of ETFs (exchange-traded funds) and mutual funds. They always pull out at the trough and surge in at the top. So, this is another indicator: When people start taking a lot out of the market, the market is turning. The last time was in March 2009, and we are 70% above that level. I’m almost happy to see an article like that (The Wall Street Journal piece) because when everyone has become optimistic or pessimistic, then you know you have reached a top or a bottom. It’s exactly the kind of article you see at a turn in the market. The prevailing opinion is always wrong.

Knowledge@Wharton: What about the idea that the economy is at a “new normal” – with low growth ahead for years?

Siegel: I don’t believe in the “new normal.”  Yes, we are in a period of low growth. I understand that, but I don’t think it will last more than a year or two. But even if the slowdown lasts two to three years, and even if (returns) go up only 1% or 2% or 3%, or even if the market is flat, these are great values. And if you believe in the “new normal,” what does that mean you should own? See what happened to gold – and commodities have broken down. Stocks are by far the most attractive investment on a valuation basis. Even if there is no growth, stocks are at the cheapest levels they have ever been — only in the 2009 market bottom, the worst bear market in 75  years, did you get cheaper stocks, when everyone thought we would be in a Great Depression.

Knowledge@Wharton: A lot of analysts worry about a double dip recession and what that might mean for stocks.

Siegel: I don’t think there is going to be a double dip, but even if there is one, recessions are temporary. So even if the markets were to decline 25%, there is a lot of downside protection because you would not be buying at the top or at an inflated price.

Knowledge@Wharton: Others point to big worries about what happens in Europe — what that might mean for the world economy and how that might affect stock markets. What is your view?

Siegel: I don’t think there will be a crisis in Europe. But, even if there is some sort of disturbance, the dollar will rise relative to other currencies. It will not cut earnings for U.S. corporations. And we would still have the emerging markets, though perhaps slowed down, growing at 4%, 5% or 6%. China would be at 6% to 8%.

Check out the Dow Jones Industrial Average all the way back to 1896 and you’ll find that September is the scariest month – not October — despite the October 1929 crash, the large market fall in October 1987 and, of course, Halloween.

The record shows that returns on average fell 1.2% during the month of September from 1896 through 2008, compared with a 0.7% gain for all other months. And if you look at the returns on a decade-by-decade basis since 1901, you’ll find a similar story. September ranks dead last – number 12 – in six out of the 11 decades, and 10^th in two others when it comes to average monthly returns. In two additional decades, it ranked eighth and ninth. That’s poor-to-entirely-dismal performances in 10 out of 11 decades.

True, there’s some ancient history factored in that data set, impressive though it is. What about more recently? Still not good. “Four times in the past decade alone, the S&P 500 shed at least 5% in September. The average September decline since 1950 is 0.6%, according to the Stock Trader’s Almanac. February is the next worst, with an average 0.2% loss, and December and November are the best, averaging 1.6% gains,” notes this article in the Huffington Post.

Why is September so bad? No one knows for certain. “I don’t know of any compelling explanation for the seasonality,” says Wharton finance professor Robert F. Stambaugh. No one seems able to point to a persuasive study offering a data-proven explanation.

And so, despite the strong pattern of selling bias in September, it could, nevertheless, simply be a random pattern – a coincidence. Still, there is no shortage of unproven theories explaining why September is a cruel month for markets.

“Some people say that September brings the reality of expenses to mind,” says Wharton finance professor Jeremy Siegel. “There are back-to-school expenses, tuition and vacation credit card bills to pay.” In some ways September is similar to January, when consumers face a spending hangover after big holiday spending.

Further afield, one theory, based on psychology, suggests the shrinking amount of daylight in September causes people to become more conservative in their spending, Siegel says. This might be akin to squirrels gathering nuts for the winter. In September, people “cash in stocks for liquidity,” and in September “daylight diminishes at a faster rate than in any other month.”

So what’s most likely to happen this month? Despite the long string of sad September stories for the Dow, Siegel thinks this time “might surprise on the upside.” And over the next four months, through year-end, Siegel would not be surprised if the Dow closed “15% higher” than yesterday’s levels.

The reasons: “I’m one of those people who thinks we will have a good last four months of the year, because I think the terrible outcomes – like recession in the U.S. and a European banking crisis — are not going to be as bad as some suggest. This is a slowdown and a disappointment – and it was not really forecast by most – but a slowdown is not the same as a recession. The private sector is still creating new jobs.”

Siegel adds that the market is already positioned for some very big shocks “that might not happen. I think the banking crisis in Europe will not happen to the degree the market fears. There may be some defaults and insolvencies,” but the problems will not “grind the banking system down to a crisis…. The ECB (European Central Bank) will come to the rescue.”