Passage Summary: People who hide their feelings—whether they mean to or not—get a fast heartbeat, so the author thinks the act of hiding the feelings is what causes the heart to race.

Conclusion: The physical act of holding back emotional displays is what triggers a significant increase in heart rate.

Reasoning: Both people who hide emotions automatically and people who hide them on purpose experience the same spike in heart rate during emotional events.

Analysis: The argument identifies a correlation between inhibition and heart rate and concludes that inhibition is the cause. However, it overlooks the possibility that the 'emotion-provoking situation' itself is causing the heart rate to rise independently of the inhibition. For the conclusion to hold, it must be true that the heart rate wouldn't jump just from the emotional stress alone. We need an assumption that rules out the external situation as the sole cause of the physiological spike.