One of the first rules we learn in voice-leading is to avoid similar motion in arriving at intervals like 5ths, octaves, unisons, and dissonances like 4ths, major/minor seconds and major/minor sevenths. The origin behind this lies in the desire to maintain the fullness of voices - when two out of four or five voices move in a similar direction to one of these intervals, the effect is one of "conflation", where two voices apparently merge into one, or at the very least lose some of their independence.
At a perceptual level, two voices that move like this together are usually perceived as harsh and cold. In Bach's times and in church music, this was to be avoided at all cost. However, this very effect of coldness and harshness can be harnessed for musical expression. Rock for example is a musical genre that draws a lot on similar motion, especially parallel 5ths.
I want to look at two examples from film music where similar motion is wonderfully used to express the harsh, cold forces in the emotional narrative of the film.
Gattaca (Andrew Niccol, 1997) is set in a future society driven by eugenics where potential children are screened through genetic diagnosis. It is the story of Vincent, whose natural breeding makes him an inferior human, and whose only chance at fulfilling his dream of spaceflight is by borrowing genetic material from Jerome, who is paralysed following an accident but has an excellent genetic profile.
The Morrow is very minimal, and the harmony could hardly be any more basic. It's a two-chord oscillating motif between two D minor and E minor chords. The interesting thing is that there is no attempt to obey the traditional voice leading rules - all 4 voices just step a major second up. Even the A steps up to B natural instead of the B flat expected from a D minor scale.
Two reasons why this works dramatically:
a) The yanking "step up' feel hints strongly at Vincent's successful spaceflight at the end of the film and the rocket lifting from the ground.
b) The cold, inhuman effect of the similar (parallel) motion in the four voices mirrors the cold, inhuman vision of a future ruled by genetic screening and eugenic ideals.
Road to Perdition (Sam Mendes, 2002) is the story of mob enforcer Michael Sullivan and his son Michael Junior on the run from his former boss, John Rooney, who has sent an assassin to kill them. The title already implies the inevitability of the tragic ending, which Thomas Newman manages to capture in his lilting, pained theme.
Here the harsh effect comes from jumping to a major second dissonance between the bass and tenor voices (Cellos/Basses and Violas respectively) by similar motion in both voices. There is no standard resolution of this dissonance. Instead, both voices step up a major second in parallel motion before falling back to the original major third interval.
The psychological appearance of this voice-leading is uncaring and violent. Quite literally, our natural humanity (the expectation of standard, flowing voice-leading) is passed over in favour of reaching a certain abstract, cold goal (mob supremacy in the drama - a chord on IV with a major second suspension in the harmony).
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