"To what extent do you agree with this statement: 'I am a narcissist.'" Scientists believe that this question could be all researchers need to make a quick and easy diagnosis of narcissism.

Publishing their study in the journal PLOS ONE, the authors claim that understanding narcissism has implications for society that extend beyond any impact on the lives of individual narcissists.

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"Narcissism is bad for society because people who are only thinking of themselves and their own interests are less helpful to others," say the researchers.

"For example, narcissistic people have low empathy, and empathy is one key motivator of philanthropic behavior, such as donating money or time to organizations," says Sara Konrath of the Indiana University Lilly Family School of Philanthropy.

"Overall, narcissism is problematic for both individuals and society. Those who think they are already great don't try to improve themselves," says Brad Bushman, co-author of the study and a professor of communication and psychology at Ohio State University.

He adds:

"And narcissism is bad for society because people who are only thinking of themselves and their own interests are less helpful to others."

The team put more than 2,200 participants of all ages through a series of 11 experiments. Analyzing their data, they found that they could reliably identify narcissists simply using the question: "To what extent do you agree with this statement: 'I am a narcissist.' (Note: The word 'narcissist' means egotistical, self-focused and vain.)"

Participants responded by rating themselves on a scale of 1 ("not very true of me") to 7 ("very true of me"). You can take the test online at Indiana University's website.

The researchers found that the self-reported answers to this question aligned "very closely" with several validated measures of narcissism, including the most widely used test for narcissism, the Narcissistic Personality Inventory (NPI), which involves 40 questions.

Single question test 'does not replace' longer questionnaires

Bushman emphasizes that the test - which the team has named the Single Item Narcissism Scale (SINS) - is not a replacement for these longer questionnaires, which provide more comprehensive information to researchers.

But he says SINS could be useful in long surveys where researchers are worried that people will become bored and drop out before they have completed. SINS may take 20 seconds to answer, while the NPI takes an average of 13.3 minutes.

"That is a big difference if you're doing a study in which participants have to complete several different survey instruments and answer a long list of other questions," Bushman says.

He explains why SINS is effective:

"People who are willing to admit they are more narcissistic than others probably actually are more narcissistic.

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."

According to the researchers, people with high narcissism scores on SINS were associated with both positive and negative outcomes.

The positives included more extraversion and slightly less depression. The negatives included more anger, shame, guilt and fear, and less agreeableness. The study reports that those with high narcissism scores also exhibited poor interpersonal relationships and "less prosocial behavior when their ego was threatened."

These people were also found to be more likely to engage in risky sexual behaviors and had difficulty maintaining long-term romantic relationships.

"We don't think SINS is a replacement for other narcissism inventories in all situations, but it has a time and place," Bushman concludes.