Jospeh V Brady's 'Executive Monkeys' (1958)
Brady first placed monkeys in 'restraining chairs' and conditioned them to press a lever. They were given electric shocks every 20 seconds unless the lever was pressed at the same time. Many of the monkeys suddenly died from ulcers caused by raised gastrointestinal hormone levels.
To test whether the ulcers were due to the shocks or to the stress, Brady used 'yoked controls'. One monkey, the 'executive, was responsible for controlling the lever to stop the shock while the second, yoked monkey could only receive the shocks, having no control over the lever. Thus, only the executive monkey had the psychological stress of making the decision - though both monkeys received the shocks.
The executive monkey on the left and the control on the right - photo copyright © 1958 Joseph V Brady
After 23 days of a 6 hours on/6 hours off schedule, the executive monkey died from a perforated ulcer. To test whether the stress might be related to the reinforcement schedule , Brady tried various routines, such as 18 hours on/6 hours off on 30 minutes on/30 minutes off. He then tested the stomachs of executive monkeys on a 6 hours on/6 hours off schedule and found that stomach acidity was highest in the rest period.
From this, Brady concluded that it was clearly stress and not the shocks which created the ulcers. He also concluded the greatest danger occurred when the sympathetic system stopped being aroused, leading to a 'parasympathetic rebound' in which the stomach was flooded with digestive hormones.
Of course, this was an experimental study with non-human animals but Brady believed the results could be applied to humans, with too much stress at work having the potential to lead to ulcers.