This paper addresses the question of why and when states join universal weapons of mass destruction (WMD) treaties. The existing research on this question deals only with the ratification of the Nuclear Non-proliferation Treaty (NPT) as the most prominent universal WMD treaty. It puts an emphasis on security cooperation with the superpowers and the participation of countries in rivalries or militarized disputes. However, those explanations do not satisfactorily explain the ratification of either the NPT or other WMD treaties. This paper puts forward a theoretical model that integrates factors connected with security costs and benefits, and social norms. The model argues that states that possess the given WMD, or search for it, are likely to postpone ratification. On the contrary, a security alliance with a major power supporting the respective treaty, support for multilateralism, and a high regional ratification rate motivate countries to ratify faster. The model also involves two counter-intuitive claims: that an insecure environment speeds up ratification, and that respect for human security rights had a positive effect on ratifications even before the ‘humanitarian turn’ in the field of arms control. To provide an empirical test for the model, the paper conducts a survival analysis of the ratifications of the five major universal WMD treaties during the years 1963-2018.