The study included three experiments. In the first experiment, the researchers used an instrument called the Dysfunctional Attitude Scale to measure the levels of pessimism in 28 mentally healthy subjects, aged 18 to 40 years. One hour later 15 of the subjects received a single dose of the compound d-fenfluramine, which is known to provoke the release of serotonin from neurons in the cerebral cortex. The remaining 13 subjects received a single dose of a control compound, clonidine, which was chosen because its side-effect profile is similar to that of d-fenfluramine. Once again, an hour later, the researchers used the Dysfunctional Attitude Scale to measure pessimism in the subjects. Those subjects who had received d-fenfluramine were found to have a significantly less-negative outlook compared with their outlook at the start of the study than was the case for subjects who had received clonidine.