Most people are familiar with the idea that popular songs are constructed chiefly of a melody (usually the lead vocal line or tune) and supporting harmonies called chords (rhythm is the other chief component). The study found that since the 1950s, there has been a decrease not only in the diversity of chords in a given song, but also in the number of novel transitions, or musical pathways, between them.
In other words, while it's true that pop songs have always been far more limited in their harmonic vocabularies than, say, a symphony, past decades saw more inventive ways of linking harmonies together than we hear now. It's the difference between Carly Rae Jepsen's "Call Me Maybe" (2012), which contains four simple chords presented one after another almost as blocks, and Alex North's "Unchained Melody" (1955), which, though also relatively harmonically simple (it employs six or seven chords, depending on the version), transitions smoothly from chord to chord due to more subtle orchestration.
The research group also discovered that timbres - the distinct textures produced by different instruments playing the same note - have gotten more homogeneous over time. This is not to say that musicians are using fewer or different instruments; rather, since 1955, pop has tended to use a smaller and more homogeneous palette of "tone colors" at a given time.
The final finding - that music recordings have grown louder and louder over time - may come as no surprise, but seems to represent the first data-driven proof. As producers compete for the attention of listeners, they have been gradually ratcheting up the inherent volume of the tracks at the cost of sound quality and dynamic richness. (You can hear this on YouTube.)
So all this study's conclusions seem plausible. But do they really mean that pop is dumber than before? To answer that, it's important to also ask what the researchers didn't study. For instance, though "Call Me Maybe" is made from a blunt and familiar set of four chords, the infectiousness of the song is in both the playful rhythmic friction between the vocals and instruments - rhythm was not taken into account in this study - as well as the cappuccino-cozy, almost country quality of Jepsen's voice, which glides and sometimes endearingly stumbles over her love-drunk lyrics.
Much musical interest in hip-pop and dance-pop derives from the pervasive four-on-the-floor dance beat - and, crucially, well-crafted rhythmic dissensions from it. ("Unchained Melody," while gorgeous, isn't known for its beat.) As tempting as it may be to try to decode the "musical discourse," as these researchers called it, there are certain aspects of music - ineffable and otherwise - that will always elude your data set.