This is an interesting question. Since economic growth varies from year-to-year, you would need to get data over a very long period (20-50 years) and average it. You would also need data on the strength of the humanities, such as the per capita rate of college graduates with degrees in the humanities.

To make this study interesting, you should propose a theoretical mechanism that could explain a causal relationship, and measure this, too. For example. Do you think economic prosperity causes more people to study the humanities? If this is true, perhaps it's because of the higher average salary of humanities majors relative to other majors in countries with greater economic growth rates. A high relative salary would thus encourage studies that are not directly related to a profession.