Supporting Online Materials An experimental study of inequality and unpredictability in an artificial cultural market

As stated in the main text, subjects entering the experiment were randomly assigned into either the independent condition or the social influence condition. Subjects in the independent condition had no information about the previous behavior of others and so were forced to make their decisions about the songs independently. However, subjects in the social influence condition were given information about the behavior of others which they could use, or ignore, when making their decisions. Any difference in success outcomes for the songs between these two groups can be attributed to presence of social influence. Our design also had an additional step. In order to better understand unpredictability, subjects in the social influence condition were further randomly assigned into one of eight influence “worlds”. Each subject was given information only about the behavior of others in their influence world. We thus created multiple “histories” to determine to what extent indistinguishable groups of subjects, starting at the same initial condition, and choosing from the same set of songs can generate different success outcomes. We only needed one independent condition realization because the behaviors of the subjects in this condition were independent. A schematic of the experimental design is shown in Fig. S1. The assignment of subjects was done such that 20% of the subjects were assigned the independent condition and 10% were assigned to each of the eight social influence worlds. For each experiments, this allocation resulted in about 700 subjects in each of the social influence worlds and about 1,400 in the independent condition. The reason for this allocation scheme will become clear when we discuss our measure of unpredictability.