Rat Park
Back home, most of them simply stopped
The hardest natural experiment there is
Lee N. Robins et al., follow-up study of returning Vietnam servicemen, 1974
Is there harder evidence than rats? What the numbers on returning soldiers actually showed.
What actually happened?
In 1971 the US government discovered an awkward problem: a substantial proportion of American servicemen in Vietnam were addicted to heroin. Everyone braced for a disaster — these men would carry it home into the cities. Robins's team followed them and found the opposite. The overwhelming majority simply stopped once home, with only a small fraction relapsing within a year. Same people, same drug, different place. It is one of the very few natural experiments in addiction research with a sample this size.
Its strength is that nobody designed it
Laboratory findings always carry the worry that they were engineered. This one wasn't — nobody sent hundreds of thousands of people to a war and brought them back in order to prove a point. Not being designed is exactly what makes it more persuasive than any experiment.
The same person, at war and back at home, responds completely differently to the same substance.
What changed wasn't the place. It was the whole situation
Coming home swapped out an entire set of conditions: no longer possibly dying at any moment, family instead of fellow soldiers, a job and a schedule, and the substance much harder to get. So changing environment isn't moving house. It's withdrawing the conditions holding it up.
Only the address changed and the hours, the circle and the pressure are identical. That isn't a change of environment.
The fraction who relapsed says something else
Some did continue after returning. That fraction is the reminder: situation explains the bulk, not all of it. Who lands in it has more to do with what home was like and what else was going on than with the drug.
If the home you return to was already an empty cage, the change of place never actually happened.
How do I use it today?
Where you are: you're trying to help yourself or a family member stop something, and you've been fighting the substance head-on.
Ask first: what conditions hold this up — hours, place, people, the gap it fills? Removing one at a time beats holding out.
Where it goes wrong: assuming a new city fixes it — what this cohort changed was an entire situation; and don't conclude that willpower doesn't matter. It says willpower shouldn't be the only term.
Lines to keep
Same people, same substance. Home again, and they stopped.
The weight of this data is that nobody designed it.
Changing environment isn't moving house. It's removing the props.