The momentum effect on the London Stock Exchange

Abstract

This study intends to investigate the momentum effect, which states that shares which performed the best (worst) over the previous three to twelve months continue to perform well (poorly) over the subsequent three to twelve months. Evidence suggests that a strategy that buys previous winner shares and sells short past loser stocks can generate abnormal profitability of about 1 per cent per month (Jegadeesh and Titman, 1993). Although momentum payoffs tend to persist when share returns in international markets are employed (e. g., Griffin et al., 2003, Rouwenhorst, 1998), a significant number of studies have debated the potential explanation of the momentum effect without reaching a consensus. Using data from the London Stock Exchange from January 1975 to October 2001, this thesis investigates some factors that influence the magnitude of continuation gains that have not been previously identified. I examine the relationship between momentum profitability and the stock market trading mechanism and is motivated by recent changes to the trading systems that have taken place on the London Stock Exchange. Since 1975 the London stock market has employed three different trading systems: a floor based system, a computerised dealer system called SEAQ and the automated auction system SETS. I find that after the introduction of the computerised dealer system SEAQ momentum profits are higher than when the floor based system operated. I also document that companies trading on the SETS auction system display greater momentum profitability than shares trading on SEAQ. Results are robust to the use of different samples and alternative risk adjustments. I investigate the role of volatility in influencing momentum profits. Shares with high volatility display wide spread out returns and therefore, potential higher magnitude momentum profitability. Given that shares displayed higher volatility traded on the post-Big Bang period (Tonks and Webb, 1991) and on the SETS system (Chelley-Steeley, 2003), I examine whether the different levels of momentum profitability achieved in alternative stock market structures arises from volatility. I find that momentum profits are strongly influenced by volatility, but the finding that the organisation of a stock market influences the momentum profits holds even after considering differences in volatility. I examine whether the magnitude of momentum profitability varies following bull and bear markets. Momentum profits stem from the winner shares in bull markets and from the loser stocks in bear markets. I report that momentum profits are stronger following bear markets, showing a sign of mean reversion in the UK stock market. Overall, this study contradicts the model of Hong and Stein (1999) that the momentum effect arises from the gradual expansion of information among investors and the model of Daniel et al, (1998) that the momentum effect stems from the investors' overconfidence that increases following the arrival of confirming news. This study also indicates that a significant portion of momentum profits stem from the magnitude of volatility.