A type of beta decay in which two electrons or positrons are given off. Maria Goeppert-Mayer predicted the possibility of this process in 1935. It was found experimentally (in the case of electrons being emitted) in 1987. Double beta decay with electrons can only occur if the binding energy of the final nucleus is greater than the binding energy of the initial nucleus and the final nucleus has an atomic number of two greater than the initial nucleus. See also neutrinoless double beta decay.