Pairs of monkeys were confined in restraining chairs and administered electric shocks at 20-second intervals. Both monkeys had identical levers and lights, which warned them when a shock was imminent. However, only one of these levers, when pressed every 20 seconds, served to prevent or turn off the shock. The “executive” monkey controlled this lever, while that of his companion (the yoked monkey) was not connected to the shock mechanism.
The yoked monkey had no control over the lever, leaving only the 'Executive' with the psychological stress of pushing the lever.
After 23 days of a 6 hours on, 6 hours off schedule to the electric shocks, the executive monkey died.
Many executive monkeys developed gastric ulcers from the stress of the electric preventing the shocks.
In comparison, the yoked monkey suffered from little stress as their behaviour had no control over the situation.
Yoked monkeys that got the same amount of shocks as their worker companion but who did not have to work on the schedule - did not ulcerate.
This is an unethical study as the monkey was not protected from harm.
Also, we may not be able to generalise findings to humans in a straightforward way.