Steptoe agrees. We’re reliant on the differing stress levels that people themselves report, he says. “And in an observational study in humans there’s always the possibility that some other factor is involved which you can’t avoid. For example, it could be that a certain type of person chooses a job that puts them under higher work stress, so any changes aren’t necessarily to do with the stress per se, but with the type of person who’s attracted to that kind of occupation.”
In the meantime, Cohen wants to know if there is evidence that other forms of ill health can be linked to stress-induced resistance in the glucocorticoid receptors. He talks of finding opportunities “to examine the role of glucocorticoid resistance in predicting the onset and progression of other inflammatory diseases, probably in epidemiological studies.” And he’s not alone. He knows, for example, of another group of researchers planning a study of the role of glucocorticoid resistance in asthma.
There is always the possibility that the definitive experiment will always remain out of reach, in which case stress researchers might find themselves in the position of doctors trying to demonstrate a causal link between lung cancer and smoking. Unable to do the definitive experiment they have had to rely on an accumulation of observational studies, animal experiments, and studies of the physiological mechanisms most likely involved.
After thirty years of exploration, Cohen says he remains undaunted by the prospect that definitive experiment might elude him, and he laughs off any suggestion of frustration. “When you work in chronic stress,” he says, “you don’t expect to be able to do experiments like that.”