One benefit suggested of the advance of knowledge about genetics is that those people who have demonstrated genetic susceptibilities will be able to take steps to avoid their fate, thus benefiting their health. A person who has demonstrated a genetic predisposition to, say, bowel cancer will attend their doctors more frequently for tests.
In a draft discussion document prepared for the European Union, for example, Beese lists the therapeutic benefits of genetic screening.
The early detection of predisposition to more frequent but also more complex genetic diseases (cancer, diabetes, hypertension) presents the greatest opportunity for medicine. ....
Several aspects of genome analysis may yield a positive impact on patient care. First, for some genetic disorders, presymptomatic knowledge of an inherited defect can provide opportunities for the use of preventive measures that minimise the morbid effects of a mutation. Second, the improved capacities to define the precise molecular defects causing a disease should markedly advance the understanding of its pathological bases, facilitating the design of rational treatments. (Beese, 1997)
The benefits foreseen are of two kinds: indirect, through the advancement of knowledge leading to new possibilities, and direct, through improved treatment. The first falls into the field of research, the second into the field of public health.
If a person is alerted through genetic screening to a danger and takes measures to avoid it, this would obviously be a benefit to that person, and, other things being equal, to the community. The field of public health has over the years found that other things are not necessarily equal. In this paper I make the case that any net treatment benefit from genetic screening depends entirely on changes in the wider health context.
The discovery of a gene predisposing its owners to death by lung cancer if they smoked, to take one example that has been raised as a possibility, would not necessarily have any impact whatsoever on the mortality or morbidity burden.
These reservations apply only to genetically-related conditions where the benefit is to the individual having the test, where precautionary measures are possible, and where genetically predisposed cases do not make up the total caseload. For the purposes of the discussion, let us take the hypothetical situation where 30% of the adult population currently smoke, where of those smokers some 10% eventually die from lung cancer, and where some 5% of the population have an additional genetic susceptibility - where they have some three times the risk of dying of that cancer. All these figures are merely illustrative, and any may be changed without affecting the argument. The significance lies in the ratios between them.
Of three thousand people who die from cancer before the discovery of the gene, then, 435 will therefore be from the 5% with the genetic condition and 2,565 from the 95% without it.
Number of Risk Number of deaths
smokers per
100,000
Before With cancer 1,500 29% 435
gene
Without 28,500 9% 2,565
cancer gene
Total 30,000 10% 3,000
Once the discovery has been made, those with the gene now realize that
for them the known risk from smoking has gone up almost three times - from
10% to 29%. If - and this is a significant assumption - smokers react in
what would be 'rational' for them as a group, their smoking rate would
drop by two-thirds. In using the term 'rational'I am not suggesting that
it can ever be rational to smoke, only that once having taken the decision
to smoke or not smoke based on one's reactions to any perceived level of
risk there will be a reason to reconsider that decision if risk levels
change.
Number of Risk No. Death
smokers per
100,000
After With cancer 517 29% 150
gene
Without 31,667 9% 2,850
cancer gene
Total 3,000
If non-smokers, however, react similarly, then they will note that their risk has gone down from 10% to 9%, a fall of 10%, and 10% more of them will take up the habit. A 10% increase in the larger group cancels out the effect of a two-thirds fall in the smaller group, and the number of deaths will remain constant. I am not suggesting that a small change in risk levels will be in itself the only contributing factor to an individual's decision to smoke or not to smoke. Any such change would be significant - might be detectable - only at the population level. Similar considerations apply, however, to small changes in pricing levels, and the effect of these has been studied extensively.
A similar analysis can be applied to other possible disease screening processes. A person who has been told he has a 'diabetes gene' or an 'obesity gene' may take more care with his diet; a person who has been told he does not may take less; there may be no net gain.
There are, of course, an enormous number of other factors that will almost certainly affect the actual outcomes. Addiction to nicotine may lead some people to irrational smoking. A large increase in risk may have a circuit-breaking shock effect. A more fundamental objection, however, is that there is no reason to expect smokers to behave 'rationally' in any sense. It is not clear whether it could ever be 'rational' to smoke, and even if this was possible, it would still be highly unlikely that present smoking patterns reflect anything resembling a rational outcome, or that the reaction of the population to changes in risk would be in any sense 'rational'.
These concessions may seem to destroy any possible force or relevance in the 'rationality' argument. This is not so. Whatever its difficulties, the argument nevertheless draws attention to previously unobserved difficulties in our current approaches to genetic screening. The particular difficulty that all these approaches share is that they assume that those people whose risk is raised will act in every sense rationally while those whose risk is lowered will not. This may or may not be the case. If people do, in this sense, behave 'rationally' then there is no net benefit, thus reducing the cost-effectiveness of genetic screening. If they do not behave 'rationally' then they must be being influenced by factors other than risk, and it is then theoretically possible to produce the same effect in the population by altering those other factors rather than by carrying out the genetic screening, thus reducing the cost-effectiveness of genetic screening. If people do not increase their smoking when the odds of death fall then that is equivalent to a decision that the previous risk levels have become unacceptable - that a 10% risk, for example, is now too high. That decision cannot possibly arise out of any consideration of the outcomes of the genetic assessment, and must be externally caused. If that external cause can be identified and employed, and if the decision to regard a 10% death rate as unacceptable is widely taken on the basis of that external cause, then the death rate will fall with or without genetic screening.
Risk perception is a recognised field of study. If treatment benefits are to be gained from genetic screening if and only if perception of raised risk is a stronger motivator than perception of lowered risk, then research is surely necessary to find out whether, and in what circumstances, this is so. I put this issue forward as an important research topic.