Child and adult speakers of English have different ideas of what 'or' means in ordinary statements of the form 'A or B'. Even more far-reaching differences between children and adults are found in other languages. This tells us that young children do not learn what 'or' means by watching how adults use 'or'. An alternative is to suppose that children draw upon a priori knowledge of the meaning of 'or'. This conclusion is reinforced by the observation that all languages adopt the same meaning of 'or' in certain structures. For example, statements of the form 'not S[A or B]' have the same meanings in all languages, and disjunctive statements receive a uniform interpretation in sentences that contain certain focus expressions, such as English 'only'. These observations are relevant for the long-standing "nature versus nurture" controversy. A linguistic property that (a) emerges in child language without decisive evidence from experience, and (b) is common to all human languages, is a likely candidate for innate specification. Experience matters, of course. As child speakers grow up, they eventually learn to use 'or' in the same way as adults do. But, based on findings from child language and cross-linguistic research, it looks like certain aspects of language, including the interpretation of disjunction, are part of the human genome.