Cohen et al (1993) investigated the role of general life stress on the vulnerability to the common cold virus. 394 participants completed a questionnaire about the number of stressful events that occurred the previous year. They were also asked to rate the degree of stress and level of negative emotions. The three scores were combined together to make what Cohen called a stress index. The participants were then exposed to the common cold virus and 82% became infected. The results showed that the chance of developing a cold was significantly correlated with stress index. Cohen et al concluded that life stress and negative emotions reduce the effectiveness of the immune system.
However a criticism of this study would be that there was no direct manipulation of the independent variable which was the stress index, so a cause and effect relationship cannot be confirmed.
Also, it’s not easy to tell if whether the participants who reported sick was due to stress or other extraneous variables.
Kiecolt-Glaser et al (1984) also investigated the impact of life stressors on the immune system. The participants were 75 medical students who were preparing for final exams. The natural killer (NK) cell activity was measured to use as an index of immune function. Measures of NK cell activity were recorded from blood samples 1 month before exams and one during exam period. Participants also completed a questionnaire on experience of negative life events and social isolation. The results showed that NK cell activity was significantly lower on high stress blood samples in comparison to low stress samples. Additionally, the greatest reductions were in students who had higher levels of social isolation. Kiecolt-Glaser concluded that examination stress reduces immune function thus making people more vulnerable to illnesses and infections.
A criticism of this study would be that there was no manipulation of the