Answer:
The answer of the human body to stress, and stressful situations, including to conditions that may trigger the stress response of the body, is a pretty complex one involving the nervous system as a whole, and the adrenal glands, which are responsible for producing two of the most important stress-related hormones. These hormones: glucocorticoids and corticoids (cortisol), will in turn affect all body systems in different manners to produce the necessary effects, to respond to the perceived "threat".
In essence, we have a pathway from our brains, to our adrenal glands, known as the hypothalamic-pituitary-adrenal (HPA) axis. In this axis, several genes that are geared towards stress responses activate (CRHR1 and CRHR2, among others) and produce CRH (Corticotropic releasing hormone), which in turn is directly related with stimulating the production of cortisol when there is stress. However, because our bodies are not meant to be in a constant state of alertness and preparation, because the different organs of the body are negatively affected, there are also pathways that will turn off this response and lower the stressful production of cortisol.
In this particular person, although he has been exposed to the same stressor as the others, he does not have precisely the same reactions to it as the others. It is important to note one thing here, and that is, that the stressor offered to these people were videos of stressful situations, and what is known about the stress response is that one vital part of it is visual exposure to different factors, including light, and images. These exposures cause the HPA axis to initiate a response through CRH production. However, it seems that in this young man, although most of the responses are present, not all of them are. So one solution could be that he has a mutation in his genes in his HPA axis, and therefore CRH is not being produced in enought levels to stimulate sufficient levels of cortisol production. However, there are also other genetical reasons why, and these could be many.
