"Time on the Cross" by Robert William Fogel and Stanley L. Engerman (1974)

For centuries, the abolition of slavery had seemed both politically infeasible and economically calamitous. Reason, gods, and violence had been employed by vested interests in order to defend the status quo. The present-day institutional details that may seem unjust and inefficient in a hundred years from now may be the inter-personal inequality in the consumption of material goods (apparently required to motivate heterogeneously-endowed individuals to work) and the voters' differential regard for the welfare of their compatriots and of foreign citizens.

Today, sufficient individual incentives are provided by avoiding the threats of death and hunger. In future, even better incentives could be provided if an individual's desire to differentiate himself from others is channelled into pursuits more productive than material consumption. The acknowledgement of the equal worth of domestic and foreign citizens would require deeper arguments justifying the tolerance of cross-country consumption inequality than the present-day implicit appeals to entitlement.

The book's distinction is that it focuses on correlations, not exceptions. Using exceptions, a reader can decorate his pet theory, and deceive himself and others. The cultivation of exceptions also makes it easier for a writer to differentiate himself from his competitors. An eye for correlations distinguishes a scientist.

The authors refrain from an anthropomorphic theory ascribing the qualities of an eccentric individual to a social class. Instead, they explore individual incentives. Then, at a class level, under appropriate conditions, individual eccentricities cancel out; behavioural regularities emerge.

The slaves' better-than-free-labour and better-than-freedman material conditions need not distract from the injustice of exploitation, measured by the slaves' superior productivity, but mostly unmeasured. Liberty comes from respect for individual idiosyncrasies. Even if slavery had been comfortable for most, it would have nonetheless been inadmissible because insufferable for few.