Let's start with a true story. A colleague in the US visited her grandson in a psychiatric hospital and discovered that in his first few days there he had been drugged into a stupor. She insisted on seeing the medical staff, and three psychiatrists told her that her grandson had a bipolar disorder caused by a brain-biochemical imbalance. Quietly, but firmly, she asked what evidence they had that there was something wrong with his brain. They said his mood and behaviour indicated a serious problem. She asked how they knew this was caused by brain biochemistry. Slightly uncomfortable, they vaguely referred to research indicating that this was the case. Much more confident now, my colleague said she knew of no studies that had ever confirmed this idea, even though it is a common assumption in the psychiatric world, and quoted studies indicating that there was nothing wrong with the brains of people with all sorts of psychiatric labels. Sensing that they were dealing with someone who might have read the literature, they asked what she wanted. Her grandson was quickly transferred to a unit offering talking therapies.

My colleague didn't ask the question that all patients, relatives and advocates should ask when told that a brain-biochemical problem lies at the root of someone's low mood or strange behaviour. She could have said: "Can I see the test results, please?" The truth is that, unlike all other doctors, psychiatrists do not do physical tests to confirm their diagnoses. The reason is simple: it is not seen as necessary to prove that there is something wrong with the brains of patients diagnosed as schizophrenic, bipolar, depressed and so on. The organic hypothesis, however, remains unproven.

All sorts of claims about the physical causes of psychological problems continue to be made in the scientific literature and popular press. Professor David Cohen, a critic of psychiatry, has noted that almost all the substances operating within the human body have been investigated as a possible cause of schizophrenia. This research is leaked to the press before it has been accepted in a peer-reviewed journal and "the results are likely to be cited around the world, thought they may never make it to scientific publication". We are also well aware of the massive conflicts of interest involved in the studies that do finally appear in the official literature. Effective Health Care, in reviewing the newest so-called anti-psychotic medication, concluded: "Most relevant trials are undertaken by those with clear pecuniary interest in the results."

Some conditions - alcohol-induced dementia, for example - show real signs of brain damage. This is rare. To date, studies have failed to demonstrate that people with the commonest psychiatric diagnoses have a brain-biochemical imbalance. The studies on all the main diagnostic categories of psychiatry are plagued by a significant problem: the people being tested have almost always already received psychotropic medication, so if there is a blood, brain or liver disorder, this may have been caused by the treatment. Physical tests on people diagnosed, but not yet treated, in the psychiatric system, would be the only way to find out if there were a difference between these people and the general public. This never happens, as most people first encounter a GP who, instead of asking for blood and other tests to demonstrate a psychiatric condition, simply prescribes a psychotropic drug or refers on to a psychiatrist.

Psychiatrists, of course, do ask for tests such as CT scans on their patients, but these are to exclude the possibility of brain damage. In other words, they are checking to see if there is a real brain problem, evidence of illegal drug use or whatever. Once they have concluded that there is nothing demonstrably amiss with the patient's brain or biochemistry, they tell the person that they have a condition that results from a biochemical problem. But they don't have a test that could prove a so-called mental illness was actually organic in origin. That's why patients are not offered tests of brain chemistry.

Imagine the same situation in oncology: you are told that you look like you have cancer, offered no tests, and told you will have two operations, followed by radiotherapy and a course of drugs that makes your hair fall out. The idea is preposterous. You demand proof of your condition. An x-ray, a CT scan and blood and liver tests reveal that the diagnosis was accurate. Reassured, though still very scared, you agree to the treatment.

So next time you are told that a psychiatric condition is due to a brain-biochemical imbalance, ask if you can see the test results.

· Craig Newnes is psychological therapies director for Shropshire's Community and Mental Health Services NHS Trust. His most recent books are This is Madness Too and Spirituality and Psychotherapy, both available from PCCS Books.