Margin Constraints and the Security Market Line Petri Jylh¨a∗ June 9, 2016 Abstract Between the years 1934 and 1974, the Federal Reserve frequently changed the initial margin requirement for the U.S. stock market. I use this variation in margin requirements to test whether leverage constraints affect the security market line, i.e. the relation between betas and expected returns. Consistent with the theoretical predictions of Frazzini and Ped- ersen (2014), but somewhat contrary to their empirical findings, I find that tighter leverage constraints result in a flatter security market line. My results provide strong empirical sup- port for the idea that funding constraints faced by investors may, at least partially, help explain the empirical failure of the capital asset pricing model. JEL Classification: G12, G14, N22. Keywords: Leverage constraints, Security market line, Margin regulations. ∗Imperial College Business School. Imperial College London, South Kensington Campus, SW7 2AZ, London, UK;
[email protected]. I thank St´ephaneChr´etien,Darrell Duffie, Samuli Kn¨upfer,Ralph Koijen, Lubos Pastor, Lasse Pedersen, Joshua Pollet, Oleg Rytchkov, Mungo Wilson, and the participants at American Economic Association 2015, Financial Intermediation Research Society 2014, Aalto University, Copenhagen Business School, Imperial College London, Luxembourg School of Finance, and Manchester Business School for helpful comments. 1 Introduction Ever since the introduction of the capital asset pricing model (CAPM) by Sharpe (1964), Lintner (1965), and Mossin (1966), finance researchers have been studying why the return difference between high and low beta stocks is smaller than predicted by the CAPM (Black, Jensen, and Scholes, 1972). One of the first explanation for this empirical flatness of the security market line is given by Black (1972) who shows that investors' inability to borrow at the risk-free rate results in a lower cross-sectional price of risk than in the CAPM.