Behavioural models generally require that the investment decisions of irrational investors aggregate in a systematic way. Using a unique Australian dataset of individual investor trades I investigate the plausibility of this assumption. I find that aggregate individual investor trades do indeed exhibit strong systematic patterns, including negative feedback trading and substantial persistence. In addition the weekly cross-sectional net trades of a large number of independent retail brokerage firms are contemporaneously correlated to a remarkable extent. Thus the aggregation assumption appears plausible. However I do not find that the net trades of retail investors consistently predict future returns in a negative fashion. In fact over the period 1991-2002, the net trades of full-service brokerage clients actually positively forecast future short-term market and cross-sectional returns. While small investors do act in a highly systematic fashion, their actions may not, at least in the short run, be classed as irrational.