Dimensional Financial Advisors is a passive-investment-oriented investment firm closely associated with Eugene Fama, 2003 Economics Nobel Laureate and father of the efficient market hypothesis. Today they sent me a link to a very nice presentation by their vice president, Weston Wellington. The presentation, “Is It Different This Time?” runs about 18 minutes long and puts the present stock market gyrations into a historical perspective by reviewing some of the “bear markets” of the last 50 years. It’s not overly jargon-filled and should be pretty accessible. The present market downturn reflects concerns about the non-availability of credit, the extent and duration of the housing market downturn, and the fact that we are probably in a recession. But in the end, if you’re investing for the long term, the question you must to ask is whether you think the economy is going to completely collapse. If not, you should hang on, assuming you’re holding a diversified portfolio that fits your tolerance for risk. The presentation is marked for general use, so I don’t think I’m breaking any rules by sharing it.