This new video from New York Magazine's "The Science of Us" series is based on scientific research conducted at Indiana University and Ohio State University that resulted in the Single-Item Narcissism Scale. From a 2014 news release about that research:
In a series of 11 experiments involving more than 2,200 people of all ages, the researchers found they could reliably identify narcissistic people by asking them this exact question (including the note):
To what extent do you agree with this statement: "I am a narcissist." (Note: The word "narcissist" means egotistical, self-focused, and vain.)
Participants rated themselves on a scale of 1 (not very true of me) to 7 (very true of me).
Results showed that people's answer to this question lined up very closely with several other validated measures of narcissism, including the widely used Narcissistic Personality Inventory (consisting of 40 questions).
"People who are willing to admit they are more narcissistic than others probably actually are more narcissistic," said Brad Bushman, co-author of the study and a professor of communication and psychology at The Ohio State University.
"People who are narcissists are almost proud of the fact. You can ask them directly because they don't see narcissism as a negative quality – they believe they are superior to other people and are fine with saying that publicly."