A mechanism postulated in grand unified theories to explain why the masses of neutrinos are very much smaller than the masses of quarks and charged leptons. The seesaw mechanism envisages that the observed light neutrinos are associated with very heavy neutrinos of right-handed helicity. It was proposed by a number of physicists in the late 1970s and early 1980s and is generally regarded as the most plausible mechanism for the masses of neutrinos. Several specific implementations of the seesaw mechanism have been proposed.