I suspect you won't accept any evidence. However it is very well established that stress causes bowel movements and diarrhea. This is very easy to demonstrate in the lab. You can probably even test it on yourself. Anyway, there's not really any doubt about this. By definition, IBS-D is an abnormal number of bowel movements, so therefore if you are under stress and that stress is sufficient to cause diarrhea, then IBS-D can be caused by stress. There are likely other factors at work as well, and IBS could be caused by a range of different things, but stress is definitely one of them. If you look at the major reviews on IBS, they say the same thing.
Let me break the argument that IBS-D is an example of a pychosomatic disorder down into pieces.
"It's pretty well established that headaches, back/muscle pain, and IBS can be caused by psychological stress, for example."
What is the mechanism? Do not say cortisol, as I will shortly show why that is most likely wrong.
Sensory and cognitive stimuli might indeed cause various hormonal and neurological responses, and from there immunological ones. Congratulations on a biological explanation. It does not prove psychogenic causation. It can easily be explained by biology all the way. That is the problem with many of these explanations, they are too vague and quickly move from mind to biology without any established scientific basis.
"I suspect you won't accept any evidence." I am happy to consider scientific evidence, that is evidence that is from sound scientific methodology, and sound analysis and reasoning is applied to it. Sadly in this area of medicine, and claimed medical research, it seems lacking far too often. Like any scientific argument it would then be subject to attempts at refutation, which is a normal part of science.
"However it is very well established that stress causes bowel movements and diarrhea."
"There are likely other factors at work as well, and IBS could be caused by a range of different things, but stress is definitely one of them. If you look at the major reviews on IBS, they say the same thing."
I have trouble finding ANY reviews that say this definitively, they are all careful to say might be or possibly or could be. A few do count psychological causation as a possibility, not an established fact. Please cite a source that is available online for the claim about reviews, and another for specific research (not a review) on IBS-D. There is also a difference between exacerbation and causation. For example, its thought likely that cognitive, emotional and physical stressors might induce post exertional malaise in Myalgic Encephalomyelitis. This does not mean they are the cause. Its the underlying mechanism that is important.
The almost universal claims are about the impact of diet, with regular questions raised about the immune system and dysbiosis.
I suspect you might well find an association with cortisol and IBS-D, plus other disroders claimed to be psychogenic. These disorders involve pain and other stimuli. A cortisol response might well be a result of the problems, not a cause. Causation needs to be seperately established.
When I see these kinds of arguments I notice the same pattern -
1. Vague claims, like talking about stress without discussing mechanisms.
2. Irrelevant claims, as if a cortisol response is involved then unless the causative direction can be established it must be treated as association only.
3. Claims beyond the evidence. Many of the arguments are hypothetically possible, and sometimes are referenced as possibilities in the literature, but cannot be said to be proven. In any case the notion of something being scientifically proven is nineteenth century science. That is the century before last. All science needs to be established in a way that is potentially refutable .. if it cannot potentially be refuted it is nonscience, aka unfalsifiable, aka pseudoscience.
The Cortisol Claim
Now if the stress response is due to cortisol, and if cortisol is shown to be associated with IBS-D or some other disorder, does this mean we can reasonably infer causation?
Let me first make a caveat that other responses are involved in stress, its not all cortisol, with adrenaline as an example, but my point is that we have to be specific, so here I debunk a cortisol claim.
There is a classic high cortisol problem in Cushing's, whether the syndrome or disease version. Irritable bowel or diarrhea are not typical symptoms. If cortisol were by itself causative then Cushing's patients would have these issues at least at a substantially increased frequency. Now this says nothing about cortisol plus other factor, a problem similar to the issue that depression is clearly not due to serotonin deficiency, but that does not disprove serotonin plus some other factors in combination are not responsible.
Let me remind everyone again ... arguments about cortisol are about biology, not psychology. If the link is thought goes to cortisol goes to IBS-D then its highly speculative at best. At worst its wrong. If they want to prove a causal chain then they can do the science.
Mind-Body Dualism
My perspective on most claims that excuse this kind of reasoning based on the argument that emphasising biology is a case of dualism is typically a dualistic argument itself. If they want to argue a monistic claim then suddenly there is no need for mind at all. There is the brain, and what it does. Biological explanations do not need the notion of mind. Its dualistic explanations that require the notion of mind. I am here excluding the (to me very weird) ideas that its all mind, that physical reality is a figment.
Association or Causation?
This issue needs to be addressed with many of these arguments. I find it plausible, for example, that many patients with many of these symptoms will have elevated cortisol. That does not prove cortisol causes the problem. It might be that causation is the other way around, or a little of both directions, or other factors are involved.
If there are that many papers that show the existence of psychosomatic disorders, please name some! Even one! In every case I have investigated there are potentially other explanations, all biological. The issue that we do not know to a high certainty what the causes are do not prove psychogenic causation, at best they only raise it as an hypothesis.
This is the psychogenic fallacy/inference as defined by Sykes. I have a blog on this if anyone is interested.
Typically the arguments fail due to one or more of three reasons ... poor scientific methodology, poor evidence, or poor reasoning. Imprecise language, lack of quality references, and hyperbolic claims seem good indicators that there is a problem with the argument.
There is work at the NIMH in the USA to try to establish an evidence based psychiatric classification. I hope they succeed, and the sooner the better.
My prediction is that the more we learn about the brain, the more the notion of mind will become irrelevant. There is no doubt the brain has an impact on biology, its a biological control system, but we still do not really understand the details of brain function. We do have an overview, and are learning more year by year, but its a long difficult road. We are still learning about rather gross anatomy, with at least two new discoveries in the last couple of years.
So the future of psychiatry is a psychology/neurology hybrid, and most issues will probably fall squarely into one camp or the other. We might not really need psychiatry as its currently practised.