At least since Harry Markowitz’s work in the 1950s, diversification has been viewed as the key to an efficient portfolio that minimizes risk for a given expected rate of return. When James Tobin received his Nobel Prize in 1981 – in part for his work on the subject – he summarized portfolio selection theory in the classic fashion: “don’t put all your eggs in one basket.”

Over the years, academicians and market professionals extended this fundamental principle to the global asset universe, highlighting the benefits of going beyond simply holding a broad group of domestic instruments to the idea of international diversification. In the case of equity portfolios, they also observed that people typically hold a smaller share of foreign stocks than simple portfolio selection models prescribe. This gap between actual and model-based optimal allocations of equity portfolios has become known in finance as the equity home bias puzzle.