The studies are based on the complexity and repetitiveness of the music, in short. Did you know for instance, that most of the modern songs are created only by... 2 people? As in those 2 people are literally creating all of them, for every singer.
Yeah, okay buddy. Sure. 2 people make music. Like let's ignore the hundreds of producers making music. It's Lukasz Gottwald and Max Martin. YEP. ONLY TWO PEOPLE MAKING MUSIC.
Music doesn't need to be complex to be "good". I've heard some trash complex music, and some beautiful songs that relied on repetitiveness. Music shifts and changes with time.