Why is synthetic speech harder to remember than natural speech?

Previous research has demonstrated that synthetic speech is less well recalled than natural speech. Luce et al (1983) concluded that this was because synthetic speech increases the effort involved in encoding and/or rehearsal of presented information. Results of the experiments described here, which involved ordered recall of lists of ten words spoken in either a synthetic or a natural voice, with repetition of the words as a measure of successful encoding, indicate that most of the memory deficit with synthetic speech is due to encoding difficulties, rather than problems with item retention. There is evidence that encoding synthetic speech involves more processing capacity than does encoding natural speech, but that once it is encoded it is stored just as efficiently.