We are the first paper to analyse and confirm the existence and extent of rational informational herding and rational informational contrarianism in a financial market experiment, and to compare and contrast these with the equivalent irrational phenomena. In our study, subjects generally behaved according to benchmark rationality. Moreover, traders who should herd or be contrarian in theory are the significant source of both. Behavioural modifications or allowing risk aversion add little to performance and insight. JEL Classification: C91, D82, G14
