Definition:Coinsurance
📋 Coinsurance carries two distinct but related meanings in the insurance industry. In health insurance, it describes the percentage of a covered claim that the policyholder must pay out of pocket after satisfying the deductible — for instance, an 80/20 coinsurance split means the insurer pays 80 percent and the insured pays 20 percent until any out-of-pocket maximum is reached. In commercial property insurance, coinsurance is a contractual mechanism that requires the insured to carry coverage equal to a specified percentage (commonly 80, 90, or 100 percent) of the property's replacement cost or actual cash value, with a penalty applied at the time of loss if the insured fails to meet that threshold.
⚙️ The property coinsurance clause works as a safeguard against chronic underinsurance. If a building worth $1 million is insured for only $500,000 under a policy with an 80 percent coinsurance requirement, the insured has only purchased 62.5 percent of the required amount ($500,000 ÷ $800,000). At the time of a partial loss, the carrier applies that same ratio to the loss settlement, meaning the insured effectively self-insures the gap. Underwriters use the coinsurance mechanism to keep premiums adequate relative to the exposure; without it, policyholders could deliberately insure for far less than full value, pay lower premiums, and still recover small losses in full — a form of adverse selection that would undermine rate adequacy for the entire book. Some policies offer an agreed amount endorsement that waives the coinsurance penalty in exchange for the insurer and insured agreeing on the property's value upfront.
💡 Beyond the individual policy level, coinsurance also refers to arrangements in the subscription market where multiple carriers share a single risk by each taking a defined percentage line. This is standard practice in Lloyd's and the broader London market, where a lead underwriter sets the terms and other participants coinsure portions of the same slip. In reinsurance, quota share treaties operate on a coinsurance principle, with the reinsurer accepting a fixed percentage of every risk in the ceding company's portfolio. Understanding which flavor of coinsurance is being discussed — cost-sharing with policyholders, penalty clauses for underinsurance, or multi-carrier risk-sharing — is essential to avoiding costly misunderstandings in policy design and claims settlement.
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