To measure narcissism, researchers use the narcissistic personality inventory (NPI), a standardized test with the 40 paired statements. Respondents must choose between, for example, "I insist upon getting the respect that is due me," or, "I usually get the respect I deserve." Scores are based on points attached to specific answers, so choosing the first statement over the second would add a point.
A score of 21 reflects high narcissism, says Twenge, who also points out that the NPI isn't intended to diagnose narcissistic personality disorder. Since it's a standardized test, however, it does serve as a useful tool for tracking changes in narcissism in a large population, which she's done in her recent study.
In their paper published in the January 2010 issue of the journal Social Psychology and Personality Science, Twenge and her colleague, social psychologist Joshua D. Foster, re-evaluated the data used in prior research. They found that a large sample from University of California at Davis, which has experienced less of an increase in narcissism prevalence than other campuses across the country, skewed the results.