twenty-first-century academic fields such as marketing and behavioral finance attempt to identify and predict the rational and irrational behavior of investors. (see the work of daniel kahneman, robert shiller, vernon l. smith, and amos tversky.) driven by emotional reactions such as greed and fear, investors can be seen to join in frantic purchasing and sales of stocks, creating bubbles and crashes. as a result, herd behavior is closely studied by behavioral finance experts in order to help predict future economic crises.