The paper proposes that reduplication as a widespread morphological process has the potential ability to form polar interrogatives in natural languages. Our proposal is based on a cross-linguistic investigation, covering empirical data from Ruyuan Mien (Miao-Yao), Nuosu Yi (Tibeto-Burman), and some Chinese dialects. The emergence of reduplicative polar interrogative in Ruyuan Mien and Nuosu Yi turns out to be the result of the interplay of phonology and syntax. The reduplicative polar interrogative in Ruyuan Mien is found to be a reduced form of the A-not-A polar interrogative, derived by deleting the negative morpheme but retaining and re-linking its tone to the first conjunct. In contrast, the reduplicative polar interrogative in Nuosu Yi is formed by linking a floating mid-level tone [33], which is equivalent to an interrogative particle, to the copy of the verbal predicate. The surface reduplicative form assumed by polar interrogatives in these languages might lead language users to believe that it is reduplication that encodes interrogativity in their languages. Only when there is no tonal difference between the base and the copy in a reduplicative polar interrogative sentence can we claim that reduplication is able to encode interrogativity. This exactly happens in the Chinese dialects spoken in Huaiyin, Mianyang, and Jianli. Our proposal not only expands the functions of reduplication, but also enriches the inventory of strategies to form polar interrogatives in natural languages.