Over at Cato Unbound, I gave Greg Clark a challenge:

Name the most credible measure of idea production that isn’t at least moderately positively correlated with population.

In his reply, he pointed to Iceland:

The 300,000 people of Iceland produced 70 films (features,
documentaries etc) between 2000 and 2010, in Icelandic! I recently saw
one, Jar City, which was very well done.

I’m not satisfied.  My reply to Greg’s reply:

Greg wants to know how I would account for Iceland. The obvious
answer is that Iceland is an extreme outlier. My challenge for Greg
was, “Name the most credible measure of idea production that isn’t at least moderately positively correlated with population” – not “Name a single counter-example.”

Greg says that Iceland’s 318,000 inhabitants produced 70 films between
2000 and 2010. It’s great to be such an over-achiever. But what about
the overall correlation between annual movie production and population?
Using IMDB’s numbers
for 2003’s top fifty film-producing countries, I calculate
movie-population correlation of +.67. If you drop underperforming
mainland China from the sample, the correlation jumps to +.88. (Raw
numbers here, Microsoft Excel sheet.)

Question: If you were Greg, how would you respond to my challenge?  What is the most credible measure of idea production that isn’t at least moderately positively correlated with population?