Abstract: This dissertation examines the history of life insurance companies in the United States as sites that put science to use developing ideas about human difference and practices of discrimination. It argues that life insurance companies and the mathematicians, statisticians, and doctors they employ have had a significant impact on the development of systems of human classification and discrimination in modern America. It pays closest attention to the tools employed by life insurance companies to enable and justify their discriminatory practices, tracing their evolution over the course of the nineteenth and early twentieth centuries. The dissertation begins with the life table, a tool built from recorded mortality data and first used by British life insurers in the second half of the eighteenth century to set rates that varied with the age of the applicant. Companies in the United States adopted life tables from Britain in the early nineteenth century. Soon thereafter, life insurance companies hired actuaries to deal with life tables and to legitimate decisions that discriminated amongst policyholders in the setting of rates and distribution of company surpluses. These actuaries, like the Mutual Life of New York’s Charles Gill or Sheppard Homans, went on to construct elaborate bookkeeping systems that allowed insurers to make finer distinctions in the terms awarded their policyholders and base those distinctions on criteria other than the age of the policyholder. In the 1840s and 50s, insurers regularly applied penalties to residents in the South and West. Sectional discrimination gained support from company mathematicians who at the same time developed arguments from their analyses of company “experience” that life in the United States obeyed laws that differed significantly from those that prevailed in England or Europe. Companies anxious to increase their volume of new business following the depression of the 1870s and content to see the end of Reconstruction abandoned regional penalties in the 1880s and 90s. Increased business in the South coincided with the extension of life insurance to wage workers in the form of “industrial insurance,” both of which factors led companies to consider race in their calculations of mortality risk. When numerous state legislatures in the 1880s and 90s outlawed discrimination in life insurance rates based solely on race, many life insurance companies stopped selling life insurance to African Americans, ceding the business to companies who found ways around the antidiscrimination laws or to new African American-run insurance companies. As a result of their work justifying their companies’ actions, statisticians from two major industrial life insurers, Frederick L. Hoffman of the Prudential and Louis I. Dublin of the Metropolitan Life, became leaders in ongoing public debates over whether higher mortality rates among African Americans inhered in racial inferiority or in factors more contingent and temporary in character. While debates over the use of racial criteria continued, life insurance medical directors, such as the New York Life’s Oscar Rogers, and actuaries, like the Mutual Life of New York’s Emory McClintock, invented the “impairment” as a blanket category to encompass various individual characteristics ranging from nationality to occupation to a history of disease or to any other factor that could be correlated to increased mortality rates. The impairment subsumed older categories, including race, as a tool for ever finer and more pervasive practices of discrimination between individual policyholders. In the first three decades of the twentieth century, health reformers teamed with life insurance officers to adopt the impairment, the life insurance medical examination, economic valuations of human life, and life tables derived from insurance mortality records, all of which became important parts of America’s modern health infrastructure.