The rationality of financial markets has been one of the most hotly contested issues in the
history of modern financial economics. Recent critics of the Efficient Markets Hypothesis ar-
gue that investors are generally irrational, exhibiting a number of predictable and financially
ruinous biases such as overconfidence (Fischoff and Slovic, 1980; Barber and Odean, 2001;
Gervais and Odean, 2001), overreaction (DeBondt and Thaler, 1986), loss aversion (Kah-
neman and Tversky, 1979; Shefrin and Statman, 1985; Odean, 1998), herding (Huberman
and Regev, 2001), psychological accounting (Tversky and Kahneman, 1981), miscalibration
of probabilities (Lichtenstein, Fischoff, and Phillips, 1982), and regret (Bell, 1982; Clarke,
Krase, and Statman, 1994). The sources of these irrationalities are often attributed to
psychological factors—fear, greed, and other emotional responses to price fluctuations and
dramatic changes in an investor’s wealth. In response to the mounting evidence of departures
from market efficiency, a growing number of economists, psychologists, and financial-industry
professionals have begun to use the terms “behavioral economics” and “behavioral finance”
to differentiate themselves from the standard orthodoxy.
However, recent research in the cognitive sciences and financial economics suggest an im-
portant link between rationality in decisionmaking and emotion (Grossberg and Gutowski,
1987; Damasio, 1994; Elster, 1998; Lo, 1999; Loewenstein, 2000; Peters and Slovic, 2000),
implying that the two notions are not antithetical, but in fact complementary. For exam-
ple, in a pilot study of 10 professional securities traders during live trading sessions, Lo
and Repin (2002) present psychophysiological evidence that even the most seasoned trader
exhibits significant emotional response—as measured by elevated levels of skin conductance
and cardiovascular variables—during certain transient market events such as increased price
volatility or intra-day breaks in trend. In a series of case studies, Steenbarger (2002) also
presents evidence linking emotion with trading performance.