In Xining Chinese, especially as used by older people, free nouns are always reduplicated, as a purely formal condition without any semantic effects. We argue that the reduplication takes place when an acategorial root is merged with a null nominal categorizer which copies the phonological matrix of the root, as an effect of a condition ruling out free monosyllabic nouns. When the condition is not independently satisfied, as in a compound or derived noun, reduplication is how the condition is met. Reduplication also occurs optionally in compounds or derived nouns. In conjunction with a minimalist theory of word formation, this will be shown to predict the distribution of reduplication in various contexts. For instance, the head of a compound can be reduplicated, but not the modifier, some affixes but not others permit reduplication of the base, non-compositional compounds do not allow reduplication, and so-called ‘bound roots’ (really, bound words) are not reduplicated. The phenomenon provides very strong evidence that simple content words are made up of an acategorial root and a categorizer which is often null, but can be overt in some languages, including Xining Chinese, where it is overt in nouns by virtue of reduplication.