Researchers at the Medical University of Vienna, Austria, studied how music listening habits are influenced by two opposing factors: the desire for variety and the demand for uniformity. Its results showed that music becomes increasingly formulaic in terms of instrumentation as sales increase due to the tendency to popularize music styles with low variety and musicians who have similar skills. For example, Euro house, disco, and pop rock decreased in complexity over time and enjoyed higher average album sales, while experimental, alternative rock, and hip hop became more complex and saw an overall sales decline. The study only looked at complexity in terms of instrumentation and defined “popular” based on album sales. The researchers also acknowledge that the success of a song or album has little to do with its complexity or quality, and is more a result of social influence.