Validity of BPS model

BSP = BioPsychoSocial model.
To those who know my hatred of abbreviations, I apologise.

Is the biopsychosocial model of illness a valid theory? Is it a better model than any other? This page considers these questions. One may test a hypothesis in several ways. Are all existing observations compatible with the theory? If not, it is already disproven. Can the theory explain observations that, up to this point, have not been explained? Are predictions made by the theory confirmed? Is there a theory that makes better predictions, for example, being more accurate? A model of illness is a theory. The model sets out the relationships between variables, and it may explain observed associations and predict phenomena. There are only a few well-established public models of illness, with the biomedical model being the pre-eminent one. The biomedical model has been the best, underlying many medical advances over the last 350 years. This page will review the evidence that the biopsychosocial model of illness is a better model – or theory.

Table of Contents


So what?” After setting out the biopsychosocial model to medical students, one implicit and sometimes explicit comment is, “Well, why is this important, and what is its relevance to us as doctors (to be)?” Fortunately for my self-esteem, other students do immediately see its relevance. Nonetheless, it is an important and pertinent question, particularly when you are part-way through a six-year learning and training programme and have so much more ‘factual and scientific’ information to learn.

This page will demonstrate that the biopsychosocial model is the best compared with the alternative models of illness. That is not to say that it is perfect or answers every question. But I hope to convince you that it is a better theory when analysing illness, whether from a clinical point of view, with an individual patient or from a broader scientific point of view.

A theory is ....

I have previously referred to the holistic biopsychosocial model as a ‘model of illness’. This phrase follows the tradition within health, where people refer to ‘the medical model’, usually as something they disagree with. I also refer to it as a theory of illness. I will discuss in the next part whether it would be more appropriate to refer to it as a model of health or use some other term.

The term model refers to a theory of illness. A theory is “a system of ideas intended to explain something, especially one based on general principles independent of the thing to be explained” [Oxford English Dictionary]. So the ‘model’ is more accurately referred to as ‘the biopsychosocial theory of illness‘.

A theory is characterised by the fact that it is falsifiable (Popper) in principle. There is no way to know that a theory is complete and unable to be improved. Under these circumstances, one designs an experiment to disprove a theory and looks for natural observations that are incompatible with the idea. If either occurs, the approach is not wholly accurate. It may, nonetheless, be the only and therefore the best theory until someone develops a better one. For example, Einstein’s theory of relativity predicted observations that were incompatible with Newton’s theory and, when confirmed, his theory demonstrated its superiority. Newton’s theory was still good enough to get men to the moon and back. It was not wrong; it was incomplete.

The current dominant model is the biomedical model. It evolved over 400 years as science developed and evolved, so no one wrote a definitive description.

So what I need to show is that the biopsychosocial theory can explain aspects of illness that the biomedical model cannot explain. It is also possible that the biopsychosocial approach will predict a condition that can be (or has been) verified. The slight difficulty in this process is the absence of any definitive description of the biomedical model, other than what I have given!

Disease and illness

As will become apparent, the biomedical model is a theory of disease. People use it as a theory of illness, which has led to the problems now facing healthcare. Therefore a short interlude is needed to explain how disease and illness differ.

Disease refers to a change in the structure and/or function of an organ or organ system within the body. In systems terminology, it is a dysfunction of one of the component systems that, between them, make up a person’s body. The term usually used for abnormality is pathology. The change, which is typically a deviation from normal for the person concerned, is sometimes but not always associated with manifestations at the body level – symptoms or signs.

The biomedical theory evolved as science advanced, and studies of human structure and function, in turn, led to science advancing. It was a very successful theory.

Illness refers to the personal experience of ‘not being well’. The Oxford English Dictionary’s definition – “a disease or period of sickness affecting the body or mind” – does not capture its essence. This definition also conflates disease with illness. Many people living with a long-term disease do not consider themselves ill. Many people with long-term disabilities do not think themselves ill.

The construct of illness encompasses a variety of recognised terms. It is related to, though not the same as, the ‘sick role’; it is akin to ‘not being healthy’. Illness is close to sickness. Sickness is entirely a socially determined state because being sick leads to the possibility of being absolved temporarily of social responsibilities. Illness is a little more. Someone may feel ill and consider himself ill, while society may not agree that he is sick and can avail himself of social help.

You may read a more recent post for further discussion of these issues and the meaning of words.

Disease, sickness, and illness

These three words are often used interchangeably. On this page they are used with specific meanings and implications:

  • disease. A term referring to abnormal structure or function within the body, usually with a particular organ or organ system, but sometimes in a volume of the body, for example after trauma.
  • sickness. A socially determined state where
    • the person considers that they are sick, usually attributing it to a disease, and
    • Society considers that the person is sick, this usually bein validated on behalf of society by a healthcare professional.
  • illness. A personally determined state where the person considers themselves sick either before anyone has made a decision or, sometimes, when healthcare does not agreee.

The three states are usually congruent, but not necessarily. For example, a person with mania will consider himself well (not sick, not having an illness, and not having disease) while society and his family may feel he is sufficiently sick to require placement in a secure setting. It is arguably whether they have a disease. It is not uncommon to have a disease without being sick or ill. More importantly, many people with long-term disease and disability nonetheless are neither sick nor ill. Sometimes society wants to label them as sick, quite unjustifiably.

In other words, illness is a personally-determined state, a feeling that in some way you are not functioning as you should be, or you are experiencing something that is concerning or distressing. In contact, sickness requires society to agree that you are ill. Illness and disease are usually present together, but not inevitably. Some people may feel that they are not unwell, while society may determine that they are sick. A person who has mania could well be sick but not ill.

Biomedical model: some failures

I outlined the main characteristics of the biomedical theory of illness here. It has had great successes over the last 400 years, allowing the identification of many diseases and treatments for some conditions. As a theory of disease, it remains a strong theory.

Unfortunately, a good theory of disease is unlikely to be a satisfactory model of illness; indeed, it may be harmful. Illness is a personally determined state, often precipitated by disease, but it is possible to have a disease and not be ill, and it is possible to be ill but not have a disease. I will outline areas where the biomedical model fails and/or causes actual harm.

The biomedical model assumes that disease is either cured or leads to death. It developed in the context of identifying the cause of an illness. It was not designed to consider long-term disease states. Rather, it was a theory that enabled researchers to discover the abnormal bodily structures or functions that caused illness. Its purpose was never to understand illness itself.

Chronic disease

Chronic disease was probably quite common even when the biomedical model first evolved, and it may have increased over the next 300 years. However, it was just one of many difficulties faced for most of the population. People did not enter a sick role; they had to work and participate in society to live. Beggars may have had a chronic disease, but they were not of great concern to society at the time.

The biomedical model does not give any helpful insight into chronic disease management. Chronic disease is outside the theory.

Multiple diseases

For precisely the same reasons, the biomedical model offers no insight into managing two or more diseases, particularly when one is chronic. If the two diseases share similar symptoms, even diagnosis may be difficult. However, the primary difficulty is that it cannot help determine which disease might be contributing to a disability, nor how significant that contribution is.

The biomedical model does not give any helpful insight into managing multiple diseases. Patients with numerous diseases are outside the theory.

Psychological disorders

The biomedical model cannot explain psychologically determined disorders. The model or theory has taken two approaches to this difficulty.

Descartes provided the first when he proposed that the mind was a separate phenomenon, simply not part of the body. This proposal led, eventually, to a separation of health services into two branches: mental health services; and the rest sometimes referred to as acute health services or medical services.

The second approach developed more recently, in the twentieth century, and started to consider psychological illnesses as having a basis in altered biochemistry and/or neurophysiology. At its height, this theory led to treatments such as leucotomies and the use of a range of drugs. Indeed the apparent success of drugs was used as evidence that psychological illness was simply another ‘physical’ illness like diabetes. My year of training in psychiatry was in such a department, I had to oversee ‘insulin coma therapy’ (i.e. prolonged hypoglycaemia, over many weeks); I was taught that it was a ‘physical treatment for a physical problem’.

It has become evident that, although some drugs may give some benefit, the primary causes of much psychological illness and distress are social and contextual. Thus, although conditions such as ‘alcoholism’, anxiety, depression, and phobias are referred to as diseases, there is no good evidence to suggest that any unique and specific bodily dysfunction is the proximate cause. This comment does not deny that genetic and other bodily disorders can increase the risk of a psychological disorder.

The biomedical model has not given any helpful insight into psychological illnesses. I could argue that it has delayed the acknowledgement of the importance of social and contextual factors and psychological and behavioural treatments that are effective.

Psychiatric disorders

I am drawing a distinction, which is possibly valid (some years since I trained in psychiatry!), between psychological illnesses as above and psychiatric illnesses that involve delusional beliefs and disordered thinking. We called them ‘major psychotic illnesses’ when I trained.

As far as I understand, these illnesses are associated with significant brain structure and function changes. Indeed schizophrenia was initially termed ‘dementia praecox’, early dementia.

Thus these disorders may have a primary bodily abnormality. Nevertheless, it is still clear that social factors have a significant influence.

Although probably by accident, the biomedical model may have given some insight into psychiatric disorders, but it is unhelpful beyond the diagnosis and does not assist with management, including rehabilitation.

Functional disorders

The biomedical model cannot explain functional disorders. Indeed, the strength of attachment to the biomedical theory of disease probably explains the prolonged failure to recognise that functional conditions ‘exist’ as a legitimate category of illness. Although not named as functional illness, doctors have recognised the phenomenon for well over 150 years. Doctors gave the phenomenon many, more or less derogatory, names: hysteria, non-organic disease, shell-shock, abnormal sickness behaviour, malingering, somatisation etc.

Doctors have only considered functional disorders a valid diagnosis over the last 25-30 years, and only researched them the last 10-15 years. There are now large and rapidly growing specialist societies, websites, and research projects. Even so, there is still active resistance. For example, some patient organisations for some functional disorders still believe that their condition is actually due to some (yet to be discovered) primary cause for which there will be some (yet to be discovered) cure.

The biomedical model’s contribution to functional disorders has been to delay their recognition and scientific investigation for, possibly, over 100 years. It still contributes to the unwillingness of many patients and clinicians to accept the reality of the condition. Many patients are not benefitting from treatments that could help, but not ‘cure’, them.


This section has shown that the biomedical model of illness fails as a model of illness. I am also suggesting that adherence to this model delayed the development of a better understanding of common psychologically-based illnesses. Further, the model is still responsible for some patients failing to gain access to, or use if offered, interventions that may reduce their distress and disability.

Holistic biopsychsocial theory of illness

I will now put forward some evidence to show that the biopsychosocial theory of illness is better than the biomedical model. It may not be perfect, but I know no better approach for use in healthcare. There may be complex sociological theories, but they will focus primarily on sociology. Healthcare needs an approach that is philosophically sound and is practically useful. Notably, any theory of illness needs to recognise the central role of disease, not as the only relevant factor, but a significant factor nonetheless.

Psychological and functional illnesses

The holistic biopsychosocial theory is based on general systems theory. George Engel emphasised this aspect of the model in his original paper. One of the characteristics of systems is that they can malfunction even when none of the individual components is malfunctioning.

What is the first thing you do when your computer freezes? Turn it off and restart it. It works. There was no fault in any part of your complex machine, yet it suffered a malfunction. Dysfunction happens in complex systems.
Hospitals are highly complex systems. Malfunctions, up to and including so-called ‘never events’, arise not because any person acts wrongly or intends to harm. The system just malfunctions despite complex control mechanisms.

People are massively more complex than computers and healthcare systems. The surprise is that we function so consistently and so well. It should not surprise anyone that our bodily ‘system’ sometimes malfunctions a little – we call it an illness. The majority of malfunction can be traced back to disease. Many diseases such as tumours are themselves due to system failures within the body. Some conditions are due to external agents, such as a virus.

Research evidence suggests that about 25% of people referred to hospitals as out-patients have a functional illness. Other population-based evidence shows that most people experience brief alterations in sensation or other functions without explanation. They are so common that we dismiss them as normal. Yet they are probably transient system disturbances.

Psychological disorders – anxiety, depression, phobias, even eating disorders – are also common. Indeed, the experience of many of these ‘disorders’ is a common day-to-day experience and part of life for most people. Only occasionally are they considered beyond ‘normal’ and become an illness.

Research also shows that social and personal adverse events or experiences are associated with an increased risk of functional disorders and psychological disorders. One plausible explanation is that these illnesses arise when the demands upon the body (including the brain and psychological functions) exceed capacity when malfunction (illness) occurs. Past experiences and genetic differences may reduce resilience, making dysfunction more likely.

Whatever the explanation, the holistic biopsychosocial model predicts psychological and functional illnesses will occur is powerful evidence supporting its validity.

Severity of disease v. severity of illness.

It is a common experience, fully confirmed by research, that severity of disease and tissue damage is only weakly related to the severity of an illness, as rated by disability or any other measure. For example, the relationship between the size of a cerebral infarct in stroke as assessed acutely does not predict outcome well. Two people with similar fractures (and other injuries) may have different consequences.

This discrepancy is difficult to explain in the biomedical model. The holistic biopsychosocial theory of illness predicts it and can easily explain it. There are many factors that moderate the effect of disease severity: family and financial support, employment, experience, other illnesses and injuries, personal strengths and weaknesses and so on.

A further specific example of validity arises from research into the INTERMED. The INTERMED is a measure of complexity explicitly constructed using the biopsychosocial illness theory. Research over 20 years has found that complexity, as measured using the INTERMED, influences long-term outcome and service use in ways that one would expect from this model.

These two observations show that factors outside disease severity, but present within the biopsychosocial theory, relate to the severity of a patient’s illness in ways that the theory would predict.

Use of the theory

Theories are usually validated through use and utility; a good idea is used more than less good theories. In healthcare, many documents, policies, and organisational structures are written or developed without explicitly using any model. The biomedical model likely has a strong influence in that case.

There are now many examples of the use of the biopsychosocial theory of illness:

  • the World Health Organisation used it to structure the International Classification of Functioning, Disability, and Health;
  • many (UK) guidelines use it, for example for stroke management, multiple sclerosis and traumatic brain injury;
  • it is used when designing and describing most rehabilitation research;
  • its use by the clinical team is mentioned in many research studies showing effectiveness of rehabilitation;
  • many ‘core data sets’ for specific conditions are being developed using the biopsychosocial theory framework to structure data;
  • measures and assessment protocols, discharge summaries, and other patient letters and reports are increasingly structured using the concepts of this theory;
  • it is taught within some (but not all) UK medical student courses.

Personally, I have used it in my day-to-day clinical, research, management and service improvement work since about 1986. I find that patients particularly find it easy to understand.

Moreover, the most recent article I have written has been downloaded 2000 times each month since published in July 2017. Its predecessor (no longer available, having been removed by my employers when I retired) was also downloaded for seven years at about the same rate.

The widespread and increasing use of the biopsychosocial theory of illness in many aspects of healthcare suggests it is meeting a need. This use is further evidence of its validity.

Other theories

A theory enters a competitive market. I have already discussed the model I take to be the major competitor to the biopsychosocial model, the biomedical model. There are other models, which I will discuss briefly.

There are innumerable models of illness. Some 25-30 years ago, a friend reviewed nursing and other models and found many. Reviews have been published. Books of some specific models have been published. This section will not be a systematic review. It will illustrate a few generic types of model.

The social model.
This model is a theory of disability rather than illness. Its central argument is that society causes disability through (a) labelling (and stigmatising) and (b) failing to make the community suitable for people who have any impairment. It argues that people are disabled (as a verb) by society.

This theory makes many valid criticisms of the biomedical model. Society could still do much more to improve the life of people with a wide variety of impairments. The proponents of this model have influenced politics. There are now laws against discrimination and other regulations that require reasonable adjustments to be made for people who are disabled.

As a theory of disability, it is valid. As a theory of illness, it is weak because it does not consider the role of disease. It is not much focused on the psychological aspects of illness. Most importantly, the ideas are fully incorporated into the biopsychosocial model of illness.

Nursing models.
There were, and probably still are, several models used within nursing. As one might expect, they primarily centre on aspects of illness that concern nurses and nursing. They were not intended to be models of illness.

Model of Human Occupation.
This is a model developed by Kielhofner, first published in 1980. It is used mainly within the occupational therapy profession. It is a version of the biopsychosocial model of illness and shares almost all the same features. It was developed specifically in relation to occupational therapy and focuses on aspects of occupational therapy.

The main features are:


  • the understanding that health comprises a hierarchical series of complex systems
  • the acknowledgement that the environment is of crucial importance, and the environment itself is comprised of hierarchical systems
  • the concept of occupation being an interaction between a person and their environment (similar to activities and participation in the biopsychosocial model)
  • the focus on social roles and the patient having their own purpose in life.

Kielhofner’s development of the Model of Human Occupation was ground-breaking at the time, using the recently published ideas within Engel’s paper. It does not include any other ideas.

Psychiatric models.
Engel formulated the biopsychosocial model within the context of psychiatry. Other models within psychiatry have been put forward. They include:

  • the biomedical disease theory
  • the ‘content theory’, which focuses on a person’s thoughts
  • behavioural models, which focus on a person’s behaviours
  • psycho-dynamic models, which focus on emotional attachments
  • socio-cultural models, which focus on a person’s social context

All these different psychiatric models simply focus on different parts of the overall holistic biopsychosocial theory of illness.

Paediatric models.
Most of these are similar to the biopsychosocial model, taking a hierarchical-level systems theory approach. The one critical point made in paediatric models, unsurprisingly, is the importance of time, primarily as it influences the child’s development over time. Peter Halligan and I have incorporated the ‘temporal context’ into the holistic biopsychosocial theory described on this site. It was absent from the original approach and is missing from the WHO ICF classification’s model.

Other models.
There are many other models. Many are probably of historical interest only. Many focus on particular patient groups (e.g. learning disability) or professions (e.g. nursing), or other aspects of rehabilitation. Most if not all are simply unique instances of the holistic biopsychosocial theory, which, like all good theories, has evolved as weaknesses (such as the failure to acknowledge the importance of time) became evident.

Criticisms of the theory

There are also criticisms of the biopsychosocial model of illness. A quick search will find several or many such articles. The objections raised will be considered.

The articles criticising the biopsychosocial model of illness fall into two groups.

The first group is critical of the theory’s use when making clinical or other decisions about individual patients. They suggest that the model is used to disadvantage people who are ill. They often refer to its use with employment and employability.

It may well be true that individual healthcare professionals misuse the theory or misinterpret observations in the context of the approach. Nonetheless, this does not invalidate the idea any more than the development of an atom bomb invalidated Einstein’s theories. The theory helps analyse a situation. It cannot give a solution or decision.

The second group of criticisms is critical of analysing a particular condition using the theory. There are a few patient support groups related to various conditions that criticise the approach.

The grounds for the criticism put forward has two steps. The first is that the theory suggests that factors other than disease should be considered when making clinical decisions. The critics often do not agree with this proposal. The primary reason for disagreement is that the critics believe that illness can only have a single cause. They interpret the proposition that many factors can relate to the nature and severity of an illness to mean that the biopsychosocial theory denies that disease might be the initial cause of or contributing to the condition concerned.

These critics combine an unwillingness to accept that almost all illnesses contribute to the patient’s clinical state with a logical fallacy that having many factors means that disease is not a factor. These critics are wedded to the biomedical theory of illness, denying the influence of other factors.

The biopsychosocial theory of illness cannot prove or refute any hypothesis about any illness. It does not carry any implications about the cause of illness. It simply provides a systematic framework to allow a complete analysis.


In summary, the apparent criticisms do not identify any weaknesses in the theory. One category of complaint concerns the use of the approach when making clinical decisions, which could be extended to its use when writing policies and protocols. The other category of criticism arises from a misunderstanding of the nature of any theory and the logic of determining causal relationships. No approach can prove or disprove causation within the framework of the idea; that will depend upon a logical analysis of the available evidence.


My primary conclusions are:

The holistic biopsychosocial model of illness is better considered as a theory of illness.
Illness refers to the person’s experience of ill-health; disease refers to a change in the structure or function of part of the person’s body.
The biomedical model of illness is better considered as a theory of disease.
There are many illnesses and many aspects of illness that the biomedical theory of disease cannot explain in its guise as a theory of illness.
The holistic biopsychosocial theory of illness predicts and can explain all observations made about illness,
The holistic biopsychosocial model of illness has demonstrated its utility and usefulness in many settings and fields, especially in clinical practice.
There are other theories of illness, but they are all either focused on some particular aspect of the holistic biopsychosocial approach to illness or are very similar.
The criticisms of the biopsychosocial theory (a) relate to its use in making decisions or (b) are wedded to a strict biomedical theory of illness, denying the possibility that multiple factors might influence an illness. The critics misunderstand the nature of a theory, believing that it proves or disproves causation which theories cannot do.
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