The Iambic-Trochaic Law (ITL) asserts that listeners associate greater acoustic intensity with group beginnings and greater duration with group endings. Some researchers have assumed a natural connection between these perceptual tendencies and universal principles underlying linguistic categories of rhythm. The experimental literature on ITL effects is limited in three ways. Few studies of listeners' perceptions of alternating sound sequences have used speech-like stimuli, cross-linguistic testing has been inadequate and existing studies have manipulated intensity and duration singly, whereas these features vary together in natural speech. This paper reports the results of three experiments conducted with native Zapotec speakers and one with native English speakers. We tested listeners' grouping biases using streams of alternating syllables in which intensity and duration were varied separately, and sequences in which they were covaried. The findings suggest that care should be taken in assuming a natural connection between the ITL and universal principles of prosodic organisation.