The common finding of many economists is that the rate has declined to a level not seen in decades. In their working paper, however, Taylor and Wieland show that the perceived decline may well be due to shifts in regulatory policy and monetary policy that have been omitted from the research.
FAZ and FAZIT-Blog: "Leitzinsen beeinflussen Anleiherenditen, aber bestimmen sie nicht"
IMFS Working Paper No. 103
John B. Taylor und Volker Wieland
"Finding the Equilibrium Real Interest Rate in a Fog of Policy Deviations"