Definition:Aggregation clause
📋 Aggregation clause is a contractual provision found in insurance policies and reinsurance treaties that defines the conditions under which multiple related losses or claims may be grouped together and treated as a single occurrence or event. This grouping has profound financial implications: it determines whether a series of losses triggers one deductible or several, and whether recoveries fall within one policy limit or stack across multiple limits. The wording of an aggregation clause is therefore among the most consequential — and most frequently litigated — language in any insurance contract.
🔍 In practice, the clause will specify the connecting factor that ties individual losses together. Common formulations include "arising out of one event," "originating from one cause," or "attributable to a single source or occurrence." In professional liability and D&O policies, for instance, the clause might aggregate all claims stemming from a single wrongful act or series of related wrongful acts. In reinsurance, hours clauses serve a similar function for natural catastrophe covers, limiting the window during which losses from one weather system can be combined. The precise language matters enormously: slight differences in wording can shift millions in claims reserves from one treaty layer to another.
⚖️ Disputes over aggregation clauses rank among the most complex in insurance litigation, because the answer to "what counts as one event?" is rarely self-evident. The Lloyds Market Association and various industry bodies have published model wordings to promote consistency, yet bespoke language remains widespread. For underwriters and brokers, negotiating a well-crafted aggregation clause is essential to aligning policyholder expectations with the carrier's intended risk appetite. When the clause is vague or poorly drafted, it creates uncertainty that inflates legal costs and delays claims settlement — an outcome that serves nobody's interest.
Related concepts