William Baumol is an economist at NYU and was the co-author of the intro textbook that I used in college. The main theory of his that I am familiar with and would like to highlight is called "Baumol's cost disease," which sounds more gruesome than it is.
Basically, Baumol's cost disease explains why productivity grows so fast in some sectors while it lags behind in others. In particular, the term refers to the fact that it is difficult to increase productivity in labor-intensive industries, like the arts or education. To use an example from Baumol himself, it takes the same amount of musicians to play a string quartet as it did 300 years ago. Or to use an example from education, the number of students that can be taught by one teacher is the same as it was 50 years ago. In fact, it may even be less now since there is more of a focus on low student-teacher ratios.
Therefore, while other industries like manufacturing clothing and printing books become more productive due to technological innovation, service industries do not due to the fact that they are labor-intensive goods. Therefore, the costs to produce those service goods remains higher than other goods. This could be an explanation for why the prices of medical care and tuition have been increasing so much while the prices of computers and most other manufactures have been decreasing or increasing at a slower rate.
Here is an article from The New Yorker that discusses the application of Baumol's cost disease further and references a newer study that confirms these ideas.
Can you think of any other examples of industries or goods and services that suffer from Baumol's cost disease?