The number of securities class actions is down significantly, falling to well below average, according to a study by legal research firm Cornerstone Research and Stanford University Law School.

So why is this happening? Is it because of rising stock prices? Or is it the US government taking a harder line on corporate wrongdoing? Or is it because of the US Supreme Court's rulings siding with business that could make it tougher for investors to bring class-action claims against corporations? Or maybe it's all three.

According to the study, 59 federal securities cases requesting class-action status were filed in the first six months of this year. That's down 42 percent from an average mid-year filing rate of 101 from the 1996-to-2005 period. Beginning with the second half of 2005, the average semi-annual filing rate dropped to 61 filings. That's 40 percent below the average observed over the preceding nine-year period. True, the losses are bigger, but the numbers are down.

Stanford Law School professor Joseph Grundfest says that this is the second year of low filing activity, which suggests we are seeing a sea change and it looks permanent.

"With increased enforcement activity by the SEC and Department of Justice, we have seen an increase in both the probability of detection and associated penalties," Professor Grundfest said. "Economic theory suggests these factors should lead to a decline in the incidence of fraud-exactly what we have seen occur since the middle of 2005. Consistent with the 'less-fraud' hypothesis, plaintiff counsel have been unable to identify any publicly traded firm that should, in the last two years, have been sued for fraud, but has not been sued. In my opinion, the size of class action securities fraud litigation activity may have experienced a permanent shift."

Because it's permanent, he does not believe the drop in litigation has anything to do with the woes of class action attack dog Milberg Weiss.

The study however does not rule out that the downturn might be a function of a strong stock market. If everyone's raking in money, people are less likely to sue. But if that theory is right, we might expect the lawsuits to start coming in again if there is a significant downturn.

At the very least, what we are seeing is a temporary shift. The market will tell us the rest.