They tested the hypothesis that heroin is addictive by itself, the one supported by the self-administration experiment, and got pretty unexpected results. As far as the scientific method is concerned, their experiment looks ok to me.
It's interesting to note that the criticism of the Rat Park experiment uses exactly the same reasoning Rat Park designers used against self-administering experiment, namely that one of seemingly innocuous parts of experimental setup (isolation and genetic variance respectively) was causing a major results bias.
It's interesting to note that the criticism of the Rat Park experiment uses exactly the same reasoning Rat Park designers used against self-administering experiment, namely that one of seemingly innocuous parts of experimental setup (isolation and genetic variance respectively) was causing a major results bias.