“Late adolescents and college students love themselves more today than ever before,” Dr. DeWall, a psychologist at the University of Kentucky, says. His study covered song lyrics from 1980 to 2007 and controlled for genre to prevent the results from being skewed by the growing popularity of, say, rap and hip-hop.

Defining the personality of a generation with song lyrics may seem a bit of a reach, but Dr. DeWall points to research done by his co-authors that showed people of the same age scoring higher in measures of narcissism on some personality tests. The extent and meaning of this trend have been hotly debated by psychologists, some of whom question the tests’ usefulness and say that young people today aren’t any more self-centered than those of earlier generations. The new study of song lyrics certainly won’t end the debate, but it does offer another way to gauge self-absorption: the Billboard Hot 100 chart. The researchers find that hit songs in the 1980s were more likely to emphasize happy togetherness, like the racial harmony sought by Paul McCartney and Stevie Wonder in “Ebony and Ivory” and the group exuberance promoted by Kool & the Gang: “Let’s all celebrate and have a good time.” Diana Ross and Lionel Richie sang of “two hearts that beat as one,” and John Lennon’s “(Just Like) Starting Over” emphasized the preciousness of “our life together.”

Today’s songs, according to the researchers’ linguistic analysis, are more likely be about one very special person: the singer. “I’m bringing sexy back,” Justin Timberlake proclaimed in 2006. The year before, Beyoncé exulted in how hot she looked while dancing — “It’s blazin’, you watch me in amazement.” And Fergie, who boasted about her “humps” while singing with the Black Eyed Peas, subsequently released a solo album in which she told her lover that she needed quality time alone: “It’s personal, myself and I.”

Two of Dr. DeWall’s co-authors, W. Keith Campbell and Jean M. Twenge, published a book in 2009 titled “The Narcissism Epidemic," which argued that narcissism is increasingly prevalent among young people — and possibly middle-aged people, too, although it’s hard for anyone to know because most of the available data comes from college students.

For several decades, students have filled out a questionnaire called the Narcissism Personality Inventory, in which they’ve had to choose between two statements like “I try not to be a show-off” and “I will usually show off if I get the chance.” The level of narcissism measured by these questionnaires has been rising since the early 1980s, according to an analysis of campus data by Dr. Twenge and Dr. Campbell.