Trochees tend to be even, iambs are usually uneven. Since Hayes (1985) it is believed that this distinction has a basis in an extralinguistic principle of rhythmic grouping:
- Elements contrasting in intensity naturally form groupings with initial
- Elements contrasting in duration naturally form groupings with final
It is believed that this ‘iambic/trochaic law’ reflects a universal cognitive tendency. But new research in musical theory seems to put this into question: adherence to the iambic/trochaic law seems to be partly dependent on the native language of the speaker. A group of researcher led by Aniruddh Patel found that speakers of (American) English conformed to the Iambic/Trochaic Law, but speakers of Japanese do not (see this summary in New Scientist). They argue that this difference in judgement is based on a difference in the syntactic structures of the languages in question, and in consequence that musical (rhythmic) perception is based at least partly on grammar. I suppose this puts into question the argument on the ‘groundedness’ of the iambic trochaic law.