Professor Bruce McEwen outlines the environmental, genetic, and experiential factors that can cause tolerable stress to become toxic.

I think it has to do with internal factors. For example, early life experiences can deprive a person of adequate self esteem, make them anxious if they did not have a stable home environment as they grew up. There are also genetic aspects; people who have the alleles of certain genes are, perhaps, more prone towards anxiety, towards mood disorders. At the same time, these genes actually wouldn’t be in the gene pool if they didn’t have some adaptive advantage, and so, for these people, they may be a little more edgy people, who may be more adventuresome, for example, and yet more vulnerable to suffer some of the consequences of this. Then, of course, the third aspect is, as we go through our lives, we have positive or negative experiences. We may, if we lose a job or have a tyrannical boss or a bad relationship, we are going to be sensitized to future relationships or future jobs. If, on the other hand, things go very well, we have a good sense of self esteem and so on, and we have a good social network, because a social network provides a buffer, whether it’s friends, family, and even in the worst of situations, people going out seeking professional help from a psychologist or psychiatrist. These are all hugely effective in helping to maintain this balance, and helping stress to become tolerable, as opposed to toxic.