From p. 119 of William H. Tucker's The Cattell Controversy: Race, Science, and Ideology:

Instead of the term genocide, which he wanted to reserve only for "a literal killing off" of all the members of a group, Cattell proposed the neologism genthanasia, for the more sensitive process of "phasing out" a "moribund culture...by educational and birth control measures, without a single member dying before his time."