The NHS has reported that one of the major causes of stress is feeling overwhelmed by a situation and thus feeling out of control.
It's important to recognise the applications of this explanation, because control is a part of so many activities relating to stress, including work, life events, and hassles. It also suggests why people with physical illnesses such as cancer feel more stressed, and why we get stressed when we're not sure what's going on.
One study which looked at lack of control as a cause of stress was Geer and Maisel. The study was a laboratory experiment using a sample of 60 students, who were made to look at photographs of dead car crash victims.
The sample was split into three conditions, each with a varying amount of control. Group 1 were told the timings of the photographs so they knew when they'd appear and disappear, and they were also told how to get rid of the photographs from the screen. Group 2 knew the timings only, and Group 3 were unaware of timings and how to stop the photographs remaining on-screen.
Stress response was measured physiologically by two measures: an ECG machine measuring heart rate (though this measurement was discarded) and through galvanic skin response, which is essentially the change in your skin's ability to conduct electricity due to an emotional response, such as fear or stress.
Group 1 had the lowest stress according to the GSR, and Group 3 had the highest, suggesting that lack of control can increase stress, and that control over your environment could help to reduce stress.