A lot of these supposed studies can't be reproduced and are just bunk. It's a result of the publish or perish busy body model. And most of these studies show the results that the people want it to show.
If there is an epidemic, it's an epidemic of useless studies.
Mapping the scale of the narcissism epidemic: Increases in narcissism 2002–2007 within ethnic groups - https://www.sciencedirect.com/science/article/pii/S009265660...
Egos Inflating Over Time: A Cross-Temporal Meta-Analysis of the Narcissistic Personality Inventory - http://onlinelibrary.wiley.com/doi/10.1111/j.1467-6494.2008....
And then there are those who question whether the Narcissistic Personality Inventory is even a good measure of narcissism at all
Addressing criticisms of the Narcissistic Personality Inventory (NPI). - http://psycnet.apa.org/record/2011-16529-013