[c6400sc]

Researchers compiled lyrics to songs from five musical genres (rap, country, pop, R&B, and rock) that were released between 1970 and 2020.

This is an interesting period to sample. The late 60s and early 70s were a high watermark for pop and rock music. Pop music evolved considerably from the 40s (Crooners and jazz-dominant hits), through the 50s and early 60s ("the American Songbook"), to the emergence of psychedelic rock and the cerebral singer-songwriter in the 60s. The increased prominence of genres light rock in the 70s and hair metal in the 80s would drag complexity down.

I think if they took the period 1920-2020, the trend would not be prominent.