… is from page 100 of Cass Sunstein’s excellent 2005 book, Laws of Fear  (footnote deleted):
Individuals are highly responsive to emotions expressed by others. Those surrounded by depressed people are more likely to become depressed.; those surrounded by enthusiastic and energetic people are more likely to feel enthusiastic and energetic. The particular mechanisms behind emotional contagion are not fully understood, but fear is a prime example of an emotion that turns out to be contagious. It is therefore predictable that a group of frightened people will end up becoming still more fearful as a result of internal conversations.