  The speech-to-song illusion is a robust psychological phenomenon whereby a
spoken sentence sounds increasingly more musical as it is repeated. Despite
decades of research, a complete formal account of this transformation is still
lacking, and some of its nuanced characteristics, namely, that certain phrases
appear to transform while others do not, is not well understood. Here we
provide a formal account of this phenomenon, by recasting it as a statistical
inference whereby a rational agent attempts to decide whether a sequence of
utterances is more likely to have been produced in a song or speech. Using this
approach and analyzing song and speech corpora, we further introduce a novel
prose-to-lyrics illusion that is purely text-based. In this illusion, simply
duplicating written sentences makes them appear more like song lyrics. We
provide robust evidence for this new illusion in both human participants and
large language models.
