pupil dilation

Your pupils widen when you encounter mental difficulty.

A 1964 study found that pupil diameter increases in relation to the effort required to process information.

The relationship is a positive correlation, not a direct mathematical proportion.

Your eyes reveal the labor of your thoughts.

Where this comes from

Study: Hess and Polt, 1964, Science
What the check found: The study exists and confirms the correlation, but describing it as a 'direct proportion' is a mathematical overstatement not supported by the experimental data.
Confidence: Solid, with limits

One of these every day, and every one names the study it came from. When a finding is contested, it says so.