The question is: why? A simple rise in savings compared with investment cannot explain why the relative returns on different assets might have changed. One possible answer is the global savings glut, first pointed out by Ben Bernanke in 2005. If Asian central banks with huge stockpiles of reserves are determined to invest them only in risk-free assets, their demand might push down the return on government bonds relative to equities. Prof Kopecky and Prof Taylor have a different theory: population ageing. As the baby boomers born after 1945 neared retirement, they began to convert holdings of equities into safer bonds, pushing bond yields down and holding equity yields up. That means it is not an overall glut of savings that drove interest rates so low, but a falling appetite for risk.

https://www.ft.com/content/7236fa60-0681-4c14-9758-f5be60379449?shareType=nongift