We investigate whether the levels of a stock market index contain any evidence of a behavioural bias depending on the proximity of the index level to 'psychological barriers'. These are certain index levels (usually in multiples of 100) at which the market tends to stick before breaking out either up or down. Extant behavioural finance literature has attributed this to investors' subjective perception of 'something special' about certain index levels where in fact no rational economic basis exists for such a perception. We carry out an empirical analysis of the NASDAQ Composite index and find that barrier effects are indeed present in that stock index. We employ simulation analysis to validate of our obtained results.