THE STRESSED STUDENT STUDY – KIECOLT GLASER ET AL.
The researchers wanted to know whether the immune system was influenced by external stressors such as examinations. The more T-cells in the blood stream, the more efficient the immune system.
HOW DID THEY DO IT?
Medical students gave blood at a low stress time (a month before exams) and also at a high stress time (during exams). The blood was tested for T-cell content.
WHAT DID THEY FIND?
During the exams the blood had lower NK(Natural Killer) cells, meaning that at times of high stress the immune system was less efficient at fighting disease.
The research is correlation. The study only says that stress and illness are related in some way.
There may be a sampling bias – different people respond to stress in different ways.
It is a natural experiment, so whilst it is high in ecological validity, there may be extraneous variables influencing the results.