The Authors have studied the voices of twins by means of an intrageminal identification test. They have recorded the voices of 104 twin-pairs (58 MZ and 46 DZ), asking each twin to listen to the play-back of his own and his co-twin's voice, urging him to indicate which of the two voice samples was his co-twin's.
In 66% of the MZ pairs both twins were unable to identify their co-twin's voice; in 21% of the MZ pairs only one twin identified his co-twin's voice, and in 13% of the MZ pairs both twins could identify their co-twin's voice. In the case of the DZ pairs the corresponding results are completely reversed, as they are respectively 2%, 20% and 78%.
This experience shows that the voice of MZ twins carried over the air is concordant and practically identical in its acoustical effect. Furthermore, the comparison of this result with the one afforded by the voice of the DZ pairs proves that the mechanisms charged with originating the human voice are highly conditioned by heredity.