# Is there a real example in which a correlation finally leads to the discovery of a non-trivial causal relationship?

More specifically, I am wondering if there is such an example satisfying the following criteria:

1. The example happened after 1888, it would be better to be after 1900—I think few people have the conception of correlation before Galton's 1888 paper.
2. The correlation found in the example is not easy to be investigated, but the correlation is non-trivial so that it is published first; then that correlation is investigated further, maybe with much more effort, and is finally confirmed as a result of a non-trivial causal relationship.
3. The correlation of $$A$$ and $$B$$ is a direct result of the causal relationship between $$A$$ and $$B$$, and not induced by a common cause $$C$$.

I understand that correlation is not causation. Basically, I want to have examples showing that the investigation of correlations sometimes leads to the discovery of really non-trivial causal relationships, and that the conception of correlation really help for those pursuing causality.

• "I think few people have the conception of correlation before Galton's 1888 paper." Paging 18th century Scottish philosopher David Hume… – Alexis May 19 at 19:06
• @Alexis Pardon me. Do you mean Hume has the conception of correlation? Or ask for some references? – Eli4ph May 20 at 1:38
• Hume is frequently credited with persuasively arguing the concept that correlation is not causation – Alexis May 20 at 3:45
• @Alexis I see. Thanks for referring Hume. I didn't know that he had been considering the conception of correlation so long before. – Eli4ph May 20 at 3:50
• @Alexis By the way, do you know any example showing that the conception of correlation and the use of a quantitative measure for correlation is indispensable for the discovery of a causal relationship? – Eli4ph May 20 at 3:58