Critics like Dr. Arnett see a number of problems with Dr. Twenge’s work. They say the test on which much of her research is based, the Narcissistic Personality Inventory, is inherently flawed — better designed to measure feelings of confidence and self-worth than actual narcissism. They also accuse her of focusing too much of her work on students at research universities, who they say are not representative of their generation.

And some critics are even more emphatic: they say the data, if collected and read correctly, simply show no generational difference in narcissism. “We calculated self-esteem scores from 1976 all the way up to 2006,” said Brent Donnellan, a psychologist at Michigan State University, referring to his and colleagues’ 2010 study using data from an annual national survey of high school students called Monitoring the Future, “and we didn’t see much difference at all.”

Dr. Twenge, who defends her work fervently, says the only reason she chose to focus on narcissism in the first place was that she followed the data. “The truth is I just started studying generations and tried to get my hands on as many scales and as much data as possible,” she said, “and that’s the theme that emerged.”

A Change Over Time

A Minnesota native and a childhood tomboy, Dr. Twenge had once planned on a career in gender studies. As a senior at the University of Chicago in 1993, she asked some classmates to take the Bem Sex Role Inventory, a 1971 survey that uses gender stereotypes to classify personalities as masculine, feminine or otherwise. What she found was surprising.

“Fifty percent of the women were scoring as masculine,” she said, far higher than the test manual considered normal. After repeating the study at the University of Michigan, where she attended graduate school, and getting similar results, she concluded that the first test wasn’t an anomaly, and looked for a way to further study the phenomenon.

Dr. Twenge decided to dig up as many old studies using the Bem survey that she could find, average out their scores by year and chart them over time. “I found that across all the studies from the ’70s to the ’90s, there was a very clear upward trend in women scoring higher on this measure of stereotypically masculine traits,” she said.

Thus a method was born. By analyzing the results of a survey that had been administered regularly to college students for decades, Dr. Twenge had found a novel way of tracking personality changes across generations — an elusive metric among social psychologists. She would eventually term her method “cross-temporal meta-analysis.”