It’s connected to the complexity of the songs:
We show that changes in the instrumentational complexity of a style are related to its number of sales and to the number of artists contributing to that style. As a style attracts a growing number of artists, its instrumentational variety usually increases. At the same time the instrumentational uniformity of a style decreases, i.e. a unique stylistic and increasingly complex expression pattern emerges. In contrast, album sales of a given style typically increase with decreasing instrumentational complexity. This can be interpreted as music becoming increasingly formulaic in terms of instrumentation once commercial or mainstream success sets in.
Lenika Cruz reads through the research:
Perhaps most interesting is the study’s tracking of “complexity life cycles.” For one, “experimental,” “folk,” and “folk rock” consistently maintained high levels of complexity through each time period studied. Others weren’t so lucky: “Soul,” “classic rock,” and “funk” started out high on the complexity scale but have since plummeted.
At different points in time, styles such as “euro house,” “disco,” and “pop rock” decreased in complexity, but enjoyed higher average album sales, while “experimental,” “alternative rock,” and “hip hop” became more complex, but saw overall sales decline. “This can be interpreted,” the researchers said, “as music becoming increasingly formulaic in terms of instrumentation under increasing sales numbers due to a tendency to popularize music styles with low variety and musicians with similar skills.” (In terms of instrumentation being the key here—and the study only looked at complexity factors that lent themselves to quantitative analysis such as acoustics and timbre).