The number of new US securities class action filings has fallen below historical averages for the fourth consecutive six-month period, a trend which is starting to look like a permanent shift, according to a new report.

For the two-year period beginning July 2005, the average semi-annual filing rate was 61, 40% below the average over the preceding nine-year period, according to the report by Stanford University's Law School, in partnership with a US legal research business Cornerstone Research.