Categories and networks in jazz evolution: The overlap between bandleaders’ jazz sidemen from 1930 to 1969

• Review of Peterson's process model to explain the shift from folk to pop to avant-garde under the light of social networks.
• From 1930 to 1949, mainly high volumes of recording sessions yields increased odds of sidemen overlap.
• From 1945 to 1959, style similarity yields increased odds of sidemen sharing.
• Race emerges as an important factor in explaining sidemen overlap, although it fades away during the 1960s.
• Intra-generational collaboration shifts to inter-generational collaboration, as styles become more fragmented and a stronger predictor of association among musicians.

This paper expands on Peterson's process model for historical changes in jazz. Peterson suggests that, given certain circumstances, musical genres migrate from ‘low-brow’ to ‘high-brow’. I test this proposition for jazz by investigating whether bandleaders were associated through the same sidemen (‘sidemen overlap’) across time, and the underlying logics leading to these overlaps. I confirm Peterson's model to the extent that sidemen overlap shifts from a ‘commercial’ logic to a ‘style-based’ logic. From 1930 to 1949, sidemen overlap between bandleaders is mainly predicted by recording session volumes (akin to ‘commercial logic’). From 1945 to 1969, style similarity emerges as an important predictor of sidemen overlap. I extend Peterson's process model by providing a more nuanced account, based on social networks. I show substantive collaboration across styles. As a consequence, stylistic shifts are not as abrupt as originally depicted. I also explore how past associations become increasingly stronger in terms of explaining sidemen overlap. Furthermore, race emerges as an important variable in explaining the same phenomenon. During the thirties, non-African-American homophily is high. After this period, African-American homophily increases steadily until the late fifties, decreasing again during the sixties.