Price's Northampton mortality table overestimated mortality, a directional bias good for insurers and adverse to annuity buyers

Richard Price's Northampton mortality table — the survivorship data behind Observations on Reversionary Payments (1771) and the actuarial standard for roughly a century — is generally described as having overestimated mortality: it implied that people died sooner than they actually did. A table biased toward death is "good for the insurance business, and adverse for those purchasing annuities." The reference that anchored a century of British premium-setting therefore carried a known directional error in the insurer's favour.

The mechanism is one of incidence. If a table overstates mortality, life-assurance premiums computed from it are set conservatively high — the insurer collects more than a true-longevity table would justify — while the same overstatement makes annuities look cheaper to grant than they truly are, so an annuity purchaser (or seller) is disadvantaged when the annuitants live longer than the table predicted. The bias is usually attributed to defects in the underlying Northampton parish data, such as in-migration and baptism records inflating the apparent count of deaths relative to births.

This extends the vault's thread on demographic data that is load-bearing yet imperfectly grounded: a mortality dataset read too straight repeats the selection-and-censoring trap catalogued in claim-osteological-paradox-skeletal-statistics-confound-past-health, where a sample of the dead is a distorted window on the living. It also rhymes with the mispricing that ran through early actuarial history before age-rating — the flat-rate annuities that Dodson's age-scaled premiums were built to correct.

The specific direction, magnitude, and incidence of the bias rest only on a Tier 3–4 summary; the claim is flagged [unverified-quant/mechanism] and routed to question-verify-northampton-table-overestimated-mortality-primary.

Addendum (2026-07-25). The first empirical check on Northampton from assured-lives data came in about 1811, when the Amicable Society's registrar compiled its own mortality experience and found it "more favourable than that shown by the Northampton Table" (claim-pensam-1811-amicable-experience-first-assured-lives-mortality-data). Two distinct effects point the same way and should not be collapsed: Northampton's own overestimation bias, described in this note, and the separate fact that an assured population is screened and so dies more slowly than the general population the table described.