All three studies were experimental within and between subjects" designs. They took place at the Utrecht University between the years 1998 and 2000. There is an intimate relationship between these three successive studies that show insight into the research process. While many of the variables are consistently similar throughout each report, the building upon knowledge is evident.
The intentions of the three studies are focused upon the relationship of dietary manipulation effecting mood on high stress-prone individuals through an experimental design. Each study acknowledges the relationship between serotonin deficiency and depressive mood among high stress subjects, and that the elevation of the neurotransmitter serotonin and its precursor tryptophan may prevent depressive mood.
Each study, through random selection, drew from a population of social-science students at the Utrecht University. The potential subjects filled in the Inadequacy Scale of the Dutch Personality Inventory measuring personal details, e.g. gender, age, and psychological history. This scale is also a measurement for neuroticism. "Neuroticism is closely related to negative affectivity, the disposition to see events as alarming and to experience aversive emotional states and also to stress proneness" (Markus, et al., 1998, p. 51). Subjects that score high for neuroticism tend to experience stress (Markus, et al., 1998, p. 51). .
Based on the findings of the scale, individuals from the highest quartiles and lowest quartiles were chosen. On average each study had seven men and seventeen women selected for the high stress (HS) group and seven men and eighteen women selected for the low stress (LS) group. The average ages of the subjects ranged from 17 to 34. All individuals selected were of normal weight for height. Considerations were also given for oral contraceptive use due to effects it may have upon cortisol responses.
