The present paper is meant to question the contradictory nature of the relativistic praxis which seems to characterise the common exercise of politics in Western contemporary democracies, causing — possibly far more efficiently than any form of theoretical relativism — the ongoing spread of ethical nihilism. Only Cass Sunstein’s Nudge Theory will be here considered, since its implicit relativism comprises a paradigmatic example of the criterion taken for granted in the design of many contemporary social policies. In our view Western societies are thereby directed toward a gradual slide into a real «dictatorship of relativism» as a pervasive cultural paradigm. All of which is prone to increase the vulnerability of democracy even to totalitarian measures. We will conclude that only behavioural principles grounded in a universal standard of truth allow the achievement of the ideals of justice and goodness which any human society claims to pursue.