󰀳

r s o s . r o y a l s o c i e t y p u b l i s h i n g . o r g

R . S o c . o p e n s c i .

󰀲

: 󰀱 󰀵 󰀰 󰀰 󰀸 󰀱

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

material, M5). The musical words from both lexica were then combined into 8

+

8

=

16 ‘topics’ using latent Dirichlet allocation (LDA). LDA is a hierarchical generative model of a text-like corpus, in which every document (here: song) is represented as a distribution over a number of topics, and every topic is represented as a distribution over all possible words (here: chord changes from the H-lexicon, and timbre clusters from the T-lexicon). We obtain the most likely model by means of probabilistic inference (electronic supplementary material, M6). Each song, then, is represented as a distribution over eight har mon ic top ics (H- top ics ) tha t cap tur e cla sse s of cho rd cha nge s (e. g. ‘do min ant -se ven th cho rd cha nge s’) and eight timbral topics (T-topics) that capture particular timbres (e.g. ‘drums, aggressive, percussive’, ‘fe mal e voi ce, mel odi c, voc al’ , der ive d fr om the exp ert ann ota tio ns) , wit h top ic pr opo rti ons

q

. Th es e to pi c frequencies were the basis of our analyses.

󰀳. Results

󰀳.󰀱. The evolution of topics

Between 1960 and 2010, the frequencies of the topics in the Hot 100 varied greatly: some topics became rarer, others became more common, yet others cycled ( ﬁgure 2 ). To help us interpret these dynamics, we made use of associations between the topics and particular artists as well as genre-tags assigned by the lis tene rs of Las t.f m, a web -ba sed mus ic dis cov ery ser vic e wit h app ro xim ate ly 50 mil lio n use rs (el ect ro nic supplementary material, M8). Considering the H-topics ﬁrst, the most frequent was H8 (mean

±

95% CI:

¯

q

=

0.236

±

0.003)—major chords without changes. Nearly two-thirds of our songs show a substantial (

>

12.5%) frequency of this topic, particularly those tagged as

CLASSIC COUNTRY

,

CLASSIC ROCK

and

LOVE

( online tables ). Its presence in the Hot 100 was quite constant, being the most common H-topic in 43 of 50 years. Ot he r H- to pi cs we re mu ch mo re dy na mi c. Be tw ee n 19 60 an d 20 09 , the me an fr eq ue nc y of H1 de cl in ed by about 75%. H1 captures the use of dominant-seventh chords. Inherently dissonant (because of the tritone interval between the third and the minor-seventh), these chords are commonly used in Jazz to create tensions that are eventually resolved to consonant chords; in Blues music, the dissonances are typically not resolved and thus add to the characteristic ‘dirty’ colour. Accordingly, we ﬁnd that songs tagged

BLUES

or

JAZZ

have a high frequency of H1; it is especially common in the songs of Blues artists such as B.B. King and Jazz artists such as Nat ‘King’ Cole. The decline of this topic, then, represents the lingering death of Jazz and Blues in the Hot 100. The remaining H-topics capture the evolution of other musical styles. H3, for example, embraces minor-seventh chords used for harmonic colour in funk, disco and soul—this topic is over-represented in

FUNK

and

DISCO

, and artists such as Chic and KC & The Sunshine Band. Between 1967 and 1977, the mean frequency of H3 more than doubles. H6 combines several chord changes that are a mainstay in modal rock tunes and therefore common in artists with big-stadium ambitions (e.g. Mötley Crüe, Van Halen, REO Speedwagon, Queen, Kiss and Alice Cooper). Its increase between 1978 and 1985, and subsequent decline in the early 1990s, marks the age of Arena Rock. Of all H-topics, H5 shows the most str iki ng cha nge in fr equ enc y . Thi s top ic, whi ch cap tur es the abs enc e of ide nti ﬁab le cho rd str uct ur e, bar ely features in the 1960s and 1970s when, a few spoken-word-music collages aside (e.g. those of Dickie Goodman), nearly all songs had clearly identiﬁable chords. H5 starts to become more frequent in the late 19 80 s an d th en ri se s ra pi dl y to a pe ak in 19 93 . Th is re pr es en ts the ri se of Hi p Ho p, Ra p an d re la te d ge nr es , as ex em pl iﬁ ed by th e mu si c of Bu sta Rh ym es , Na s an d Sn oo p Do g, wh o al l us e ch or ds pa rt ic ul ar ly ra re ly ( online tables ). The frequencies of the timbral Topics, too, evolve over time. T3, described as ‘energetic, speech, bright’, shows the same dynamics as H5 and is also associated with the rise of Hip Hop-related genres. Several of the other timbral topics, however, appear to rise and fall repeatedly, suggesting recurring fashions in instrumentation. For example, the evolution of T4 (‘piano, orchestra, harmonic’) appears sinusoidal, suggesting a return in the 2000s to timbral qualities prominent in the 1970s. T5 (‘guitar, loud, energetic’) underwent two full cycles with peaks in 1966 and 1985, heading upward once more in 2009. The second, larger, peak coincides with a peak in H6, the chord changes also associated with stadium rock groups such as Mötley Crüe ( online tables ) . Finally, T1 (‘drums, aggressive, percussive’) rises continuously until 1990, which coincides with the spread of new percussive technology such as drum machines and the gated reverb effect famously used by Phil Collins on

In the air tonight

, 1981. Accordingly, T1 is over-represented in songs tagged

DANCE

,

DISCO

and

NEW WAVE

and artists such as The Pet Shop Boys. After 1990, the frequency of T1 declines: the reign of the drum machine was over.