It is a setting worthy of Agatha Christie. The corpse of a thrifty pensioner is lying on the floor of a country house, shot through the head, the fingers of one hand wrapped around the butt of a revolver and a bank statement clutched in the other. The cause of death is obvious. But who fired the gun? Was the small saver murdered? Or did she pull the trigger herself?

That is the question raised (metaphorically) by the economists Joseph Kopecky and Alan Taylor in a recent paper called “The Murder-Suicide of the Rentier”. It tries to untangle why risk-free interest rates have collapsed during the past couple of decades, and finds an answer with important implications for both the economy and the returns that stock market investors can expect in the years to come.

The title of their paper is a play on a comment by John Maynard Keynes, who predicted, in 1936, the “euthanasia of the rentier”. As technology advanced, populations declined and the capital stock grew larger, Keynes imagined that interest rates would dwindle to the point where rentiers — traditional coupon-clipping investors, living off interest in their country homes — could no longer survive.