Robert Greene's claim: narcissists and toxic people can fake nearly every non-verbal signal — including a convincing smile — except genuine life in their eyes. Their eyes stay "dead" and look through a person as a "selfobject" (something to be used) rather than at them as a person. This is framed as the one signal that survives otherwise skilled deception, making it a diagnostic bottleneck for vetting someone's real intentions.
The broader mechanism: narcissists deceive by projecting a charming, alluring front that draws a target into their world before exploiting or hurting them — see CAVA: Control, Approval, Validation, Attention (Manipulator Goals) for the underlying control/approval/validation/attention goals, and Empathy's Limit: Infection from Toxic People for the risk of over-extending empathy toward someone giving off this tell. Reading dead eyes requires the same underlying discipline as Body Language Reveals What Words Conceal — muting the words and watching the face directly rather than trusting stated intentions — and is the harder-to-fake counterpart to Genuine Smile vs. Fake Smile Detection.
Apply: when assessing whether someone's warmth or interest is genuine — especially after other signals (a smile, flattering words, charm) have checked out — look specifically at their eyes for a lack of life or connection, a sense of being looked "through" rather than "at." Treat this as a stronger red flag than any verbal reassurance.