Answer:
Endowment effect
Explanation:
According to my research on studies conducted by various psychologists, I can say that based on the information provided within the question this is referred to as the Endowment effect. This effect (like mentioned in the question) explains the phenomenon in which individuals place an increased value and are more likely to keep an object than go an obtain that object if they didn't have one. Which is what is happening in this situation with the key chain.
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