When faced with entities with potentially ambiguous category membership, adult category judgments are strongly biased toward dangerous and distinctive properties. For example, a cyanide-water mixture is categorized as cyanide. We used a developmental approach to better understand this cross-domain effect, which we term the asymmetric categorization of mixtures (ACM). According to ACM, attention is biased toward perceived dangerous or distinctive properties, making them prominent in conceptual representation. We consider whether ACM is driven entirely by low-level processes of attention, or whether ACM might require the integration of attention with causal-explanatory reasoning. We argue that ACM requires forms of reasoning that only emerge robustly in middle childhood. Across three studies (N = 270), we find that ACM emerges only after the 7th year for liquids (Studies 1 through 3), and even later for race (Studies 1 and 3). Results are discussed in terms of competing theoretical accounts of ACM.