The SINS won’t work in all environments. There are environments where narcissism is seen as extreme egoism and as something bad, condemnable, something to overcome. For instance, within Mahayana Buddhism, e.g. Tibetan Buddhism, there is a strong emphasis on empathy or compassion which is seen as an ideal one should strive for. A narcissist living in such an environment will adopt this ideal and emulate to live in it while he or she still has not overcome his or her narcism. Instead the person uses the ideal of empathy/compassion to increase his narcissism (or to feed it), thinking he or she would be a great compassionate person, demanding admiration and recognition for his or her great compassion and "self-lessness".

If you do the SINS with such a person, I doubt that such a person will agree to a question »"I am a narcissist." (Note: The word “narcissist” means egotistical, self-focused, and vain.)« though he or she is a narcissist because this question does not conform to the sense of being a grandiose, compassionate, non-egoistic person. I assume in such a context this SINS will rather totally fail.

The SINS doesn’t take into account that there are other social environments where this test might fail to a high percentage.

As a reminder: »Even altruism and love itself can be nothing more than elaborate façades built to protect and disguise the ego.« – François de La Rochefoucauld (17th century)