Barbara Lust, James Gair. Lexical Anaphors and Pronouns in Selected South Asian Languages. Page 637. Walter de Gruyter, 1999. ISBN 9783110143881.
Geeti Sen. Crossing Boundaries. Orient Blackswan, 1997. ISBN 9788125013419. Page 132. Quote: "Possibly, Punjabi is the only major South Asian language that has this kind of tonal character. There does seem to have been some speculation among scholars about the possible origin of Punjabi's tone-language character but without any final and convincing answer."
Dediu, Dan; Ladd, D. Robert (2007). "Linguistic tone is related to the population frequency of the adaptive haplogroups of two brain size genes, ASPM and Microcephalin", PNAS Early Edition, retrieved on 2007-06-12.