Is it not self evident? A subject doesn't have to be particularly difficult to impact how you think. Being exposed to algebra provides you with new ways of looking at the relations between things. Being exposed to even basic world history gives you perspective that you wouldn't otherwise have had about society at large. I've observed the effect in both myself and those around me to the extent that it appears self evident to me.
Unfortunately any of the obvious metrics you might use to quantify this would seem to be hopelessly biased. The more advanced the degree someone holds the farther above average that individual tends to be in various ways. I doubt it would be possible to control for such large cross correlations.
Perhaps asking people who learn multiple languages could provide subjective evidence since that doesn't have nearly as much of a correlation with other abilities.
Unfortunately any of the obvious metrics you might use to quantify this would seem to be hopelessly biased. The more advanced the degree someone holds the farther above average that individual tends to be in various ways. I doubt it would be possible to control for such large cross correlations.
Perhaps asking people who learn multiple languages could provide subjective evidence since that doesn't have nearly as much of a correlation with other abilities.