Depressive and anxiety symptoms are often experienced together, both in people who are otherwise healthy, and in those with mental illness. They share a similar heritability: a family history of depression makes you more susceptible to anxiety, and vice versa.
We also know that there is another cluster of symptoms that isn’t specific to either depression or anxiety, but is often experienced alongside them. These relate to feelings of stress and inner-tension; of feeling wound-up and on-edge.
There has been some debate about whether the three clusters of symptoms (depression, anxiety, and stress-tension) arise from the same underlying “negative affect” (feeling bad) variable, or whether there are causal relationships between the symptoms. In other words, are the symptoms simply different manifestations of the same thing, or does one set of symptoms cause the others?
We have just published a paper that examines the nature of the relationships between the three sets of symptoms. We used a statistical approach that takes advantage of a key characteristic of identical twins. Identical twins are similar in many respects, including that they show strong correlations in measures such as their experience of negative affect. Not only that, but when we measured the three subtypes of negative affect – using the Depression Anxiety Stress Scales – we could see that the amount of depression in one twin was strongly related to the amount of stress in the other (and similarly for the other relationships).
This allowed us to use some statistical tricks to see whether one set of symptoms “caused” another. The term “caused” using this approach, means that if we were to experimentally increase the levels of, say, stress–tension, then we would expect to see an increase in the level of depression (and if there was no causal relationship, we wouldn’t see an effect).
And this is what we found. Stress–tension “caused” depression and anxiety, and depression also had a causal effect on anxiety. Our modelling shows that these symptom clusters do not simply arise from the same underlying factor, but suggests that stress has a particularly fundamental role in negative mood states.
In some respects this isn’t surprising – we all know that stress is bad for us. But our finding that stress drives depression and anxiety is important. It suggests that as clinicians we should be placing more emphasis on reducing stress, on finding strategies for our patients to reduce their internal tension. Not only is this good in itself, but it might also reduce depression and anxiety. Conversely, specifically focusing on anxiety symptoms might not do much beyond temporarily reducing anxiety. It won’t influence levels of stress and inner tension, which will continue to drive the anxiety (and depressive) symptoms.
You can download a pdf of the paper here.