Biopsychosocial model
The biopsychosocial model is a general model or approach that posits that biological, psychological (which entails thoughts, emotions, and behaviors), and social factors (abbreviated "BPS") all play a significant role in human functioning in the context of disease or illness. This is in contrast to the traditional, reductionist biomedical model of medicine that suggests every disease process can be reduced down (i.e., explained) by an underlying process, such as an antigen.  The concept is used in fields such as medicine, nursing, health psychology and sociology, and particularly in more specialist fields such as psychiatry, health psychology chiropractic and clinical psychology. The biopsychosocial paradigm is also a technical term for the popular concept of the mind-body connection, which addresses more philosophical arguments between the biopsychosocial and biomedical models, rather than their empirical exploration and clinical application.
The "model" was theorised by psychiatrist George Engel at the University of Rochester, and putatively discussed in a 1977 article in Science, where he posited "the need for a new medical model"; however no single definitive, irreducable model has been published. However, the general BPS model has guided formulation and testing of models within each professional field (see below re: health behavior models for an example).

# Model description and application in medicine
The biopsychosocial model implies treatment of disease processes (e.g., type 2 diabetes, cancer, etc.) requires that the health care team address biological, psychological and social influences upon a patient's functioning. In a philosophical sense, the biopsychosocial model states that the workings of the body can affect the mind, and the workings of the mind can affect the body. This means both a direct interaction between mind and body as well as indirect effects through intermediate factors.
The biopsychosocial model presumes that it is important to handle the three together as a growing body of empirical literature suggests that patient perceptions of health and threat of disease, as well as barriers in a patient's social or cultural environment, appears to influence the likelihood that a patient will engage in health-promoting or treatment behaviors, such as medication taking, proper diet, engaging in physical activity 
While operating from a BPS framework requires that more information be gathered during a consultation, a growing trend in US healthcare (and already well-established in Europe such as in the U.K. & Germany) includes the integration of professional services through integrated disciplinary teams, to provide better care and address the patient's needs at all three levels  As seen, for example in integrated primary care clinics, such as used in the U.K., Germany, U.S. Veteran's Administration, U.S. military, Kaiser Permanente, integrated teams may comprise of physicians, nurses, health psychologists, social workers, and other specialties to address all three aspects of the BPS framework, allowing the physician to focus on predominantly biological mechanisms of the patient's complaints  See also 
There are also theories that the state of mind directly affects the immune system, and there are many carefully-planned studies that show this to be the case (psychoneuroimmunology). Psychosocial factors can cause a biological effect by predisposing the patient to risk factors. An example is that depression by itself may not cause liver problems, but a depressed person may be more likely to have alcohol problems, thus liver damage. It is this increased risk-taking that can also lead to an increased likelihood of disease. Most of the diseases referred to in BPS discussion tend to be such behaviourally-moderated illnesses which have known high risk factors, or so-called "biopsychosocial illnesses/disorders"  An example of this is type 2 diabetes, which with the growing prevalence of obesity and physical inactivity, is on course to become a worldwide pandemic (e.g., approximately 20 million Americans are estimated to have diabetes, with 90% to 95% considered type 2). 
It is important to note that the biopsychosocial model does not provide a straightforward, testable model to explain the interactions or causal influences (i.e., amount of variance accounted for) by each of the components (biological, psychological, or social). Rather, the model has been a general framework to guide theoretical and empirical exploration, which has amassed a great deal of research since Engel's 1977 article. One of the areas that has been greatly influenced is the formulation and testing of social-cognitive models of health behavior over the past 30 years. . While no single model has taken precedence, a large body of empirical literature has identified social-cognitive (the psyho-social aspect of Engel's model) variables that appear to influence engagement in healthy behaviors and adhere to prescribed medical regimens, such as self-efficacy, in chronic diseases such as type 2 diabetes, cardiovascular disease, etc.    These models include the Health Belief Model, Theory of Reasoned Action and Theory of Planned Behavior, Transtheortical Model, the Relapse Prevention Model, Gollwitzer's implementation-intentions, the Precaution-Adoption Model, the Health Action Process Approach, etc... 

# Criticism of the Biopsychosocial model
Some critics point out this question of distinction and of determination of the roles of illness and disease runs against the growing concept of the patient-doctor partnership or patient empowerment, as "biopsychosocial" becomes one more disengenous euphemism for psychosomatic illness. This may be exploitated by medical insurance companies or government welfare departments eager to limit or deny access to medical and social care.
Some psychiatrists see the BPS model as flawed, in either formulation or application.  Epstein et al describes six conflicting interpretations of what the model might be, and proposes that "...habits of mind may be the missing link between a biopsychosocial intent and clinical reality." Rather than the result of the BPS model, David Pilgrim suggests that neccessitous pragmatism and a form of "mutual tolerance" (Goldie, 1977) has forced a co-existence and not "genuine evidence of theoretical integration as a shared BPS orthodoxy." Pilgrim goes on to state, "Despite these scientific and ethical virtues," the BPS model "...has not been properly realised. It seems to have been pushed into the shadows by a return to medicine and the re-ascendancy of a biomedical model." 
Perhaps the most vocal philosophical critic of the BPS model, Niall McLaren (ISBN 9781932690392) writes:
"Since the collapse of the 19th century models (psychoanalysis, biologism and behaviourism), psychiatrists have been in search of a model which integrates the psyche and the soma.  So keen has been their search that they embraced the so-called biopsychosocial model without ever bothering to check its details.  If, at any time over the last three decades, they had done so, they would have found it had none.  This would have forced them into the embarrassing position of having to acknowledge that modern psychiatry is operating in a theoretical vacuum."
McLaren claims that his model of emergent dualist interaction, based in the work of Alan Turing and David Chalmers, more than satisfies the expectations institutional psychiatry has placed upon Engel's ambitious "model."