Quantitative easing has failed to promote economic activity as expected, but it has driven interest rates to virtually zero for six years. Given that backdrop, asset bubbles are to be expected, said KC Mathews, CIO and economist at UMB Bank (UMBF - Get Report) . "Clearly we are not experiencing a productivity boom similar to those that have promoted bubbles in the past. However, we are experiencing the other end of the equation-historically low interest rates," said Mathews. "I believe that a bubble is developing, caused by aggressive monetary policy around the globe." Mathews labels today's bubble "rational" because he can't provide the counter-factual argument. As to when this bubble will become irrational to the point it pops, Mathews said most asset bubbles come and go within a three to five-year period. He said defensive sectors like utilities are looking the most inflated at this point and are worth monitoring.