The Iambic-Trochaic Law ( Bolton, 1894; Hayes, 1995; Woodrow, 1909) asserts that listeners associate greater intensity with group beginnings (a loud-first preference) and greater duration with group endings (a long-last preference). Hayes ( 1987; 1995) posits a natural connection between the prominences referred to in the ITL and the locations of stressed syllables in feet. However, not all lengthening in final positions originates with stressed syllables, and greater duration may also be associated with stress in nonfinal (trochaic) positions. The research described here challenged the notion that presumptive long-last effects necessarily reflect stress-related duration patterns, and investigated the general hypothesis that the robustness of long-last effects should vary depending on the strength of the association between final positions and increased duration, whatever its source. Two ITL studies were conducted in which native speakers of Spanish and of English grouped streams of rhythmically alternating syllables in which vowel intensity and/or duration levels were varied. These languages were chosen because while they are prosodically similar, increased duration on constituent-final syllables is both more common and more salient in English than Spanish. Outcomes revealed robust loud-first effects in both language groups. Long-last effects were significantly weaker in the Spanish group when vowel duration was varied singly. However, long-last effects were present and comparable in both language groups when intensity and duration were covaried. Intensity was a more robust predictor of responses than duration. A primary conclusion was that whether or not humans’ rhythmic grouping preferences have an innate component, duration-based grouping preferences, at least, and the magnitude of intensity-based effects are shaped by listeners’ backgrounds.