Construct · 3 fields · 4 citations
One name. Different field. Different construct. One map.
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In the mathematics of interest an annuity is any series of equal payments made at fixed, regular intervals, valued by discounting each payment back to the present at a stated interest rate. Kellison's The Theory of Interest (2009) sets out the standard formulas for the present and accumulated value of such a series, distinguishing an annuity-immediate, paid at the end of each period, from an annuity-due, paid at the start. Here the number of payments is fixed and known, so the same identity prices a mortgage, a bond's coupons, or a car loan. This referent is a deterministic valuation formula for a known series, not a payout that stops at death, not a household's puzzling reluctance to buy longevity insurance, and not a legal right charged on a person.[1]
In actuarial science a life annuity is a series of payments that continues only while a named person is alive, so the number of payments is not fixed but a random quantity governed by a mortality table. Valuing it means weighting each future payment by the probability that the annuitant survives to receive it and then discounting for interest, which yields an expected present value rather than a certain one. Halley's 1693 study of the Breslau bills of mortality built the first such table and used it to compute, as its title puts it, the price of annuities upon lives. This referent is a mortality-weighted expectation over an uncertain lifetime, distinct from the fixed-term formula of financial mathematics, from the economic question of why people avoid annuities, and from the lawyer's annuity as a property right.[2]
In economics the annuity is a good that a rational saver is predicted to want and yet, in practice, mostly declines to buy. Yaari's 1965 model of a consumer with an uncertain lifetime showed that, absent a motive to leave a bequest, such a person should convert nearly all wealth into life annuities, because an annuity pays a survival premium that ordinary saving cannot match. The wide gap between that prediction and the thin market for voluntary annuities is what Davidoff, Brown, and Diamond's 2005 Annuities and Individual Welfare analysed as the annuity puzzle. This referent is a welfare claim and a demand anomaly about a financial product, not the payment-series formula of financial mathematics, not the actuarial pricing of a life-contingent stream, and not a legal right to fixed sums.[3][4]
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