https://www.sciencedirect.com/science/article/pii/S0191886915300167 n=2153 To be fair, the precise argument is not "narcissists will tell you they're narcissists" it's "asking narcissists if they're narcissists will give you as accurate an assessment as dancing around the issue with 40 different questions." Your sister-in-law probably isn't a narcissist. She might be a bitch, but she's probably not a narcissist.Consistent with their initial study, we find that the SINS correlates positively with the Narcissistic Personality Inventory (NPI) and has good discriminant validity from common measures of self-esteem. Additionally, we provide new evidence that the SINS may not primarily tap into grandiose narcissism. We also find that in comparison to other common personality measures of narcissism, the SINS correlated somewhat less consistently with our behavioral measure and has a higher threshold for detecting narcissistic traits. Overall, we conclude that when inclusion of established measures is not feasible, the SINS may be a viable alternative.