Abstract
Although, when seeking to reduce the number of reported cases of certain ailments within a human group, it may be acceptable to choose to refer to this from a socio-economic perspective as being directed at improving 'population health', a biological interpretation of this way of thinking is not so accommodating. The notion of 'population health', it will be argued, has no direct equivalent in biology; there is no group-biological phenomenon that can be called 'health' - nor, for that matter, 'disease'. The terms 'disease' and 'health' apply only to individuals and can only really be understood as descriptors of an individual's overall biological state expressed at the organismal level.
Relating individual physiological parameters to population derived standards appears to have begun in the nineteenth century when our present notion of the 'normal' seems to have emerged. However, although one may be able to combine physiological parameters to form statistical means and standard deviations, 'disease' and 'health' are not of the same kind; they are not discrete phenomena and cannot necessarily be inferred simply by reference to physiological data. Here there is a tendency to mix - and perhaps even confuse - individuals and populations. The notion that health is the typical or modal state of some statistical distribution - a view that has been described as 'naïve normalism' - pervades much of current medical and related education and is found, in one form or another, in many associated texts. However, most considerations of 'disease' and 'health' as concepts seem to take the biological nature of individuals for granted. Yet, how we understand ourselves as biological entities must influence the way in which we understand 'disease' and 'health' - whether we choose to apply these terms exclusively to individuals or whether we allow them to be applied to populations. In some biologically orientated thinking about 'disease' and 'health', an appeal to normal 'species design' has been made. It will be argued from a survey of various biological phenomena that conformity to some notion of 'species design' is not necessary in order to understand 'disease' or 'health' and that an alternative, individualised approach is possible - and indeed preferable. Accordingly, it will be suggested that instead of 'disease' and 'health' being thought of as occupying opposite ends of a linear distribution (such as the Gaussian or 'normal' distribution), a two dimensional plane may be a better representation of the state of an individual such that two separate characteristics - that of objective physical state and subjective experiential state - can be considered simultaneously.
As one who teaches a course in physical anthropology which focuses on the variability within and between different human groups, I have had, for some time, certain problems with epidemiology and related subjects. In particular, I have a problem with the way in which research findings from these disciplines tend to be reported in the media. Typical is the reporting of an article from The Lancet1 just three weeks ago (28th July 2007). It was claimed in the media that people had a 40% risk of developing psychosis if they smoked marijuana. This cannot be strictly true. In a large group of marijuana smokers, one might expect, on average, 40% of them to develop psychosis but this is not the same as individual risk. Clearly, in the example given, there were individuals who were seemingly immune to marijuana-induced psychosis, so their's was no risk at all. The impression usually given by such studies is of a homogenous population each member of which is equally prone to certain physical or mental conditions if they do, or do not, practise certain lifestyle choices. This is a false impression.
That said, I have no real problem with the use of expressions such as 'population health', 'public health' or the like, if we are clear about what we mean and false impressions are avoided. However, I am concerned that sometimes ideas that apply only to individuals and those that apply only to groups are not being kept sufficiently separate.
In Britain, every weekday morning some time between 8.35am and 8.45am, a reporter on BBC Radio tells us how the stock exchange is doing by quoting the FTSE (footsie) 100 share index and how much it has risen or fallen. The FTSE 100 does not tell us how well shares in specific companies are doing; it simply serves as an overall indicator of the mood of the market. For individual companies, there is share price to consider. Just as there are reckonings to be made about the general state of the stock market and about specific companies, so too are there reckonings to be made about groups of people and about people as individuals. However, although we may be able to count the number of reported cases of certain medical conditions in a population and be concerned or relieved when those numbers go up or down, we do not really have a biomedical equivalent of the FTSE 100 when it comes to the wellbeing of groups of individuals. There is no single simple 'Health (or Disease) Index'. The reason for this is due, in good part, to the fact that there is no such thing as an equivalent to share price for individuals. We cannot measure what is essentially the quality of an individual as an organism and come up with a numerical value. As a result, we have no figure to average or otherwise manipulate so as to give a measure of the state of groups of individuals. This, however, has not stopped us from thinking about humans and other organisms in group terms – and, to varying degrees getting side-tracked along the way.
This is not helped by the fact that while we cannot measure the quality of an organism, certain characteristics of populations, even those composed of independently-minded individuals, do indeed obey mathematical principles. In genetics, for example, the Hardy-Weinberg equilibrium is followed to the extent that we can confidently predict gene or allele frequencies within populations. Indeed, it can be used to model evolution which is now commonly viewed as a change in gene frequencies in populations. However, this says nothing about the genetic makeup of the individual per se; the particular combination of genes from which the individual results. Furthermore, even knowledge of a particular individual's genome does not give a complete determination of that individual. The action of given genes is dependent upon interactions with other parts of the individual's genome and with a web of interactions with the environment. These can affect not only the degree of penetrance of a given gene but what that gene actually does.
The model of disease and health that Christopher Boorse has suggested combines (and perhaps conflates and confuses) population-based thinking with thinking about the state of the individual. In essence, for Boorse, an individual's disease or health status can be understood by comparison with a Gaussian curve depicting the distribution of members of the population for different levels of function. Those with impaired function are deemed to have pathological states; health is the absence of such states. However, if Boorse does conflate and confuse population-based thinking with thinking about the state of the individual, he does so perhaps because he sees himself, at least in part, describing the thinking already involved in attributing to individuals the labels 'disease' and 'health' rather than prescribing how this should be done. It would appear that this type of comparison with reference values is what does frequently occur during certain clinical consultations. While comparison with functional values may not be an adequate basis for the definition of what constitutes 'disease' or 'health', it certainly influences the nature of the treatment meted out.
It was during the nineteenth century that the use of the Gaussian or 'normal' distribution curve became established and when our present notion of the 'normal' seems to have come to mean that which is most common or average. Refinements and extensions of this notion over the last century or so have meant that clinical consultations have come to be characterized by the comparison of one or more of a patient's measured physiological parameters with reference values drawn from a population standard. The notion that health is the typical or modal state of some statistical distribution - a view that has been described as 'naïve normalism' - pervades much of current medical and related education and is found, in one form or another, in many associated texts. If a measured parameter is outside a prescribed range, then the question of a patient's need of treatment arises regardless of how well the patient feels in himself or how well he is able to go about his daily living. However, biologically what matters most is the viability of the individual as an organism and how well that individual is able to survive in the world. Simply going outside prescribed anatomical and/or physiological limits is no guarantee of upsetting an individual's ability to survive.
Famously, the American basketball player Randy Foye, (who, at the time of writing, plays for the Minnesota Timberwolves), is known to have the positions of his internal organs reversed by comparison with what typically appears in textbooks - a situation known as situs inversus. However, Foye is not known to be adversely affected by this and his playing career is not expected to be affected either. Furthermore, anatomical dissecting rooms regularly throw up examples of what is atypical. These are referred to technically as anomalies and are, in effect, non-conformities with the textbooks. However, most are unlikely to have ever impinged upon the individual's life in any significant way.
The idea that species adhere to body plans - or bauplans - is questionable. Organisms do not adhere to plans – even though we are all remarkably similar and even though we often refer, rather loosely, to the genome as a 'genetic blueprint'. Embryological development does not proceed by following a blueprint in any human sense; it proceeds by various processes being turned on and off genetically at certain points. Whether all these processes have to be turned on and off in one precise sequence, I do not know. For some it seems likely; for others it seems unlikely.
Organisms only have a consistent 'designed' appearance because they are the products of processes that operate in consistent – or apparently consistent – ways. What is more, these processes do not operate independently of the environment. Alligators are a graphic example in that their sex is determined by the temperature at which they are incubated as eggs. Organisms should not be viewed as genetically planned but as the products of genetically operated processes which are affected by a range of influences during the times when they are active.
What, for example, are we to make of conjoined (or Siamese) twins? They surely bring into question any notion of design. It is important to note that the term 'conjoined' is, in fact, a misnomer. The name infers that two persons are joined or merged together. It is striking, however, that non-identical twins are never joined together; only identical twins. We now know that conjoined twins result from one fertilized ovum which does not go on to become two fully separate twinned entities. While, as people, conjoined twins may have quite distinct personalities, prior to separation, all are single biological entities which are more easily understood as the products of processes that have taken particular turns on the road to organismal viability or non-viability rather than the products of plans that have 'gone wrong' or been 'mis-read'.
Species design thinking creates a problem for the philosophy of medicine. In particular, it begs questions about whether certain anatomical or physiological states can be considered healthy or not. In respect of the examples I have already given, how should one consider those with situs inversus and conjoined twins? Are they healthy or 'anomalously healthy' or are they just biological entities that find themselves having to make their way in the world like any other being – albeit with differing degrees of success and with differing degrees of personal ease?
While there is a noticeable lack of focus in the science of biology on how individual lives are lived out, in reality it is the 'living-out' – the survival of the individual organism – that matters most. Organisms engage in two biological imperatives: acts which affect individual survival and those which affect reproduction. Of these, individual survival is of paramount importance since one cannot engage in reproduction without first having been successful at surviving.
Here, what is at issue is not anatomical or physiological conformity to some plan or to some norm but how well an organism is able to survive as an individual entity in relation to the world in which it finds itself. This may include being part of a population of other individuals but never, at least in higher animals, is a population a continuous entity into which the individual organism is subsumed losing its physical integrity.
Given their lack of focus on the individual, one aspect of life that biologists largely overlook is life as lived and experienced by the person; the dimension of how people (and other animals) feel in themselves. This is unsurprising since there is no objective scientific yardstick for measuring such a state. Perhaps the closest we can come to this is some psychological evaluation of an individual's ability to manage their lives.
As noted earlier, there appears to be a tendency exhibited in much medically-related teaching towards naïve normalism: the assumption that there is a simple relationship between physiological parameters and disease and health. As a physiological parameter varies along a single axis, then so too, it is assumed, do disease and health. Disease and health have come to be seen as occupying opposite ends of a linear continuum. One must ask whether this is an accurate state of affairs. One must question what degree of commensurability really exists between a physiologically measured parameter and the ascription of the labels 'disease' or 'health' to whole individuals. Indeed, what does being at opposite ends of a continuum mean in this respect?
This one dimensional approach is primarily physicalist in that it places particular emphasis on anatomical and physiological parameters to determine the disease or health status of individuals. This approach removes, or at best subjugates, the dimension of self-experience from full consideration. It distorts our appreciation of the significance of how the individual feels as an entity surviving in the world and, furthermore, how this may feed back into that ability to survive.
What the naïve normalist tries to do is accommodate the whole complexity of organismal state in one dimension. This seems somewhat unrealistic. Just as an individual's physical status has a bearing on their survival chances, so too does their experiential status. That is, how capable or otherwise they feel when confronted with the pressures of survival – or as we usually call it: 'daily life'. We know that we sometimes feel unwell without obvious physical basis and that sometimes we feel well despite the presence of organic lesions. At other times, feelings and physiology seem to tally. These complex relationships cannot be adequately described using the simplistic bio-statistical models currently available. The ascription of the labels 'disease' or 'health' should not only be the product of how an individual is physically but should also take into consideration self-experience.
I suggest that a better biologically-based model to describe and help us understand the bases of disease and health is one which accommodates the experiential as well as the physical. That is, not a one-dimensional linear comparison with reference data but a two-dimensional approach, which unites the physical nature of the individual with their system of self-reference, that is, their means of experiencing themselves. Even if such a model is not perfect, it would be fuller than any other such model currently found in either biology or medicine.
A two-dimensional model that depicts the state of the individual in experiential as well as physical terms has biological and medical potential. Biologically, how individuals feel within themselves is fundamental to their survival success – but this, biologists largely overlook. Medically, how individuals feel within themselves is not just an aspect of why they might consult their doctors in the first place - it is an important feature throughout the course of their treatment. By giving us a fuller depiction of the individual, such a model may help us reach a better consensus about the definition of concepts such as disease and health.
I did not come to try to bury notions of 'population (or public) health'. If we wish to apply the notion of health to populations and talk of 'Population Health' or 'Public Health', or even 'Men's Health', 'Women's Health' etc., that is fine. What I hope I have pointed to here is that one should be careful about the potential for confusion - even conflation - of ideas that belong only to individuals with ideas that belong only to populations. In so doing, I hope that I have also given some indication of what can be drawn from a scientific and philosophical approach to biology when given a medical focus.
1 The Lancet Volume 370, Issue 9584, Pages 291-360 (28 July 2007-3 August 2007)