Oddly Wired
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emotion

You adopt the mood of the room without choosing.

A 1994 research review found that people automatically mimic the facial expressions and postures of those around them.

Emotional contagion is an involuntary process. By matching the physical state of others, your nervous system begins to experience the same emotional state.

Your mood is frequently a reflection of the people you spend time with.

Where this comes from

Study
Hatfield, Cacioppo, and Rapson, 1994
What the check found
The book 'Emotional Contagion' by Hatfield, Cacioppo, and Rapson (1994) is the definitive source for this claim. The provided text lacks the quote, but the claim restates a well-established theory.
Confidence
Solid, with limits

Related

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