Fig. 1: Stimuli and experimental setup.

a Frequency spectra of example musical interval stimuli, each generated from the superposition of two harmonic notes separated by a particular pitch interval. Canonically consonant intervals such as the octave, perfect fifth, and perfect fourth produce aggregate spectra whose frequencies approximate harmonics of a common fundamental frequency (f0). The octave has the greatest overlap with the best matching harmonic series, followed by the fifth, and then the fourth. Canonically dissonant intervals such as the major second produce an aggregate set of frequencies that are consistent only with a much lower f0, and that produce little overlap with its harmonics. b Map of the region of Bolivia where testing occurred. Tsimane’ participants were residents of the villages of Mara, Moseruna, Emeya, and Donoy. c Example of experimental conditions; photo depicts an experiment conducted in a classroom. Sounds were presented over closed headphones using a laptop, and were audible only to the participant (such that the experimenter was blind to the stimulus being presented). A translator (shown here with the orange shirt) assisted in explaining the experiments and interpreting responses. d Schematic and musical depiction of consonant and dissonant musical intervals used in experiments, showing alternation of consonant and dissonant intervals as interval magnitude (difference between the note f0s) increases.