Yet, unlike most academic experiments, its treatment condition utterly changed individuals’ lives. And, unlike previous draft lotteries, the Vietnam lotteries arrived at a Goldilocks moment in the history of human science. They began just when the systematic collection of data in durable formats had taken root, but before social and behavioral scientists became so enamored with field experiments that excessive efforts to study them degraded their “naturalness.”

Now, 50 years later, the Vietnam draft lotteries have become the drosophila of the social sciences: the model organism for researchers to discern how a life-changing intervention carries implications for the individuals who experienced it, versus those who escaped it by chance.

The first investigation to treat the Vietnam lotteries as an experiment focused on an enduring public concern—the challenges facing veterans upon returning to civilian life. After U.S. troops left Vietnam, stories of veterans suffering difficult returns to civilian life spread widely and were detailed in news reports and dramatized in films such as The Deer Hunter. In the most tragic instances, veterans’ suffering resulted in death. How common was this outcome?

Eleven years after the fall of Saigon, Norman Hearst, Thomas B. Newman, and Stephen B. Hulley used their knowledge of the Selective Service Lotteries to design a study that would answer that question. They could not simply examine the correlation between service in Vietnam and mortality, because serving in the military might correlate with other factors—such as a willingness to take risks—that would independently make individuals more likely to die. Hearst, Newman, and Hulley recognized this problem and knew the solution: a randomized experiment, which assigns treatment (here, to military service) by chance.

Read: How could Vietnam happen? An autopsy

The draft lotteries worked in just this way. In each lottery, dates—representing the birthday of draft-eligible men—were randomly paired with the numbers 1 to 365 (or 366 for lotteries covering a leap year). In the first lottery, the succession of birthdates drawn from a vase determined the assigned lottery number—the first date drawn received lottery number 1; the second date, number 2; and so on. In subsequent lotteries, officials improved the randomization by simultaneously drawing numbers and birthdates from different receptacles. The number paired with each birthdate determined the order in which men were called for military induction.

This procedure made those with lower numbers more likely to face military service, not because of any personal attribute likely to be correlated with life outcomes, but because of a random draw of an innocuous attribute unrelated to much of anything—their birthdate. Indeed, that was its intention. The lottery aimed to replace a system that disproportionately forced some individuals into service with a system in which everyone had the same chance of induction. With induction assigned by chance, no correlation should exist between service and inductees’ personal attributes (social class, race, risk tolerance, and so forth).