The relationship has only held in the United States, and only for two main periods: 1921 to 1929 and from 1985 to 2000. It has been shown to be flawed on a theoretical basis, fails to hold in long-term analysis of data (both in the United States, and international markets), and has poor predictive power for future returns on a 1, 5 and 10-year basis. The relationship can breakdown completely at very low real yields (from natural forces, or where yields are artificially suppressed by quantitative easing); in such circumstances, without additional central bank support for the stock market (e.g. use of the Greenspan put by the Fed in 2020, or the Bank of Japan's purchase of equities post-2013), the relationship collapses.