The 'Iambic/Trochaic law' is that if beats contrast in length, the most prominent beat will naturally come last in the group (iambic rhythm), but if they contrast in intensity, the most prominent will naturally come first (trochaic rhythm).
An aside
This post follows Chapter 4.5 of Hayes 1995, Metrical Stress Theory. He states the Iambic/Trochaic law as a reformulation of Bolton 1894, but I got rather lost in Bolton's descriptions of primitive versus civilised poetry, and the reactions of children and savages to rhythm compared to civilised adults, and am going to take Hayes' word on it being in there somewhere. I wonder in which areas our culture shapes current linguistic thought? It's very hard to know from the inside.
An experiment for the reader
Try reading out the following. Capitals indicate 'intensity' - volume. Or if you like, pronouncing it as 'a' rather than schwa, 'uh'. Long aa indicates... a long syllable.
DA da DA da DA da DA da
da DA da DA da DA da DA
daa da daa da daa da daa da
da daa da daa da daa da daa
Now, how you group them will probably depend on which one came first. But you may find - I certainly did - that it feels more natural to have the long syllable second, or the intense syllable first. When I tried to put the intense syllable second, I found that it got longer.
Stress in English is a major part of our language, and is indicated by being both more intense and longer, so it's hard to separate them.
The most recent replication of this experiement is Rice 1992. All the various versions of it find that people tend to obey the Iambic/Trochaic law.
DA da DA da DA da DA da
da DA da DA da DA da DA
daa da daa da daa da daa da
da daa da daa da daa da daa
Now, how you group them will probably depend on which one came first. But you may find - I certainly did - that it feels more natural to have the long syllable second, or the intense syllable first. When I tried to put the intense syllable second, I found that it got longer.
Stress in English is a major part of our language, and is indicated by being both more intense and longer, so it's hard to separate them.
The most recent replication of this experiement is Rice 1992. All the various versions of it find that people tend to obey the Iambic/Trochaic law.
Music
Cooper and Meyer say that "durational differences... tend to produce end-accented groupings; intensity differences tend to produce beginning-accented groupings". Their account of rhythmic groupings in music was done according to the intuitions of musicians.
Verse
Fant, Kruckenberg and Nord found that musicians using musical notation to represent poetry obeyed this law.
They found measurable differences between the (larger) durational contrasts in readings of iambic verse and (the smaller contrasts in) readings of trochaic verse.
They also found differences in the ways that iambic and trochaic verse were written. The difference in number of phonemes between strong and weak syllables in iambic verse was much greater than in trochaic verse.
They found measurable differences between the (larger) durational contrasts in readings of iambic verse and (the smaller contrasts in) readings of trochaic verse.
They also found differences in the ways that iambic and trochaic verse were written. The difference in number of phonemes between strong and weak syllables in iambic verse was much greater than in trochaic verse.
Conclusion
So there you go. Why did Shakespeare choose to write in iambic pentameter when most words in English are trochaic1? Perhaps because he wanted to have durational contrast in his poetry, rather than a contrast of intensity.
Footnotes
1For given values of "this label is applicable to the mess that is the English stress system"References
Bolton, Thaddeus (1894) Rhythm The American Journal of Psychology Vol 6, Issue 2 pp.145-238
Hayes, Bruce (1995) Metrical stress theory: Principles and case studies. University of Chicago Press pp.79-85
Rice, Curt (1992) Binarity and ternarity in metrical theory: parametric extensions. PhD Dissertation, University of Texas at Austin
Cooper, Grosvenor and Leonard Meyer (1960) The rhythmic structure of music University of Chicago Press
Fant, Gunnar, Anita Kruckenberg and Lennart Nord (1991) Stress Patterns and Rhythm in the Reading of Prose and Poetry with Analogies to Music Performance in Music, Language, Speech and Brain, pp.380-407
Cooper, Grosvenor and Leonard Meyer (1960) The rhythmic structure of music University of Chicago Press
Fant, Gunnar, Anita Kruckenberg and Lennart Nord (1991) Stress Patterns and Rhythm in the Reading of Prose and Poetry with Analogies to Music Performance in Music, Language, Speech and Brain, pp.380-407