The Washington Post had a short article on the decline in corporate taxes paid by the Dow 30.

I have an ongoing research project with Adam Rosenzweig on the decline in U.S. corporate taxes.  We’re specifically interested in what is causing the decline and what government policies, if any, have been effective in slowing this decline.

But here is a quick picture on effective corporate tax rates of the Fortune 500 (from Compustat data).  We use the same methodology as this paper on lobbying and taxes. Below is a graph on the average effective corporate tax rates of the Fortune 500 with a trend line and the 95% confidence interval.

There clearly is a decline.  But what is the cause?  Is it learning by firms?  Changing rules on transfer pricing and use of tax havens?  Government policies that are increasingly generous in their deductions and allowances?