The Fallacy of Faulty Risk Assessment

by Tim Harding

(An edited version of this essay was published in The Skeptic magazine, September 2016, Vol 36 No 3)

Australian Skeptics have tackled many false beliefs over the years, often in co-operation with other organisations.  We have had some successes – for instance, belief in homeopathy finally seems to be on the wane.  Nevertheless, false beliefs about vaccination and fluoridation just won’t lie down and die – despite concerted campaigns by medical practitioners, dentists, governments and more recently the media.  Why are these beliefs so immune to evidence and arguments?

There are several possible explanations for the persistence of these false beliefs.  One is denialism – the rejection of established facts in favour of personal opinions.  Closely related are conspiracy theories, which typically allege that facts have been suppressed or fabricated by ‘the powers that be’, in an attempt by denialists to explain the discrepancies between their opinions and the findings of science.  A third possibility is an error of reasoning or fallacy known as Faulty Risk Assessment, which is the topic of this article.

Before going on to discuss vaccination and fluoridation in terms of this fallacy, I would like to talk about risk and risk assessment in general.

What is risk assessment?

Hardly anything we do in life is risk-free. Whenever we travel in a car or even walk along a footpath, most people are aware that there is a small but finite risk of being injured or killed.  Yet this risk does not keep us away from roads.  We intuitively make an informal risk assessment that the level of this risk is acceptable in the circumstances.

In more formal terms, ‘risk’ may be defined as the probability or likelihood of something bad happening multiplied by the resulting cost/benefit ratio if it does happen.  Risk analysis is the process of discovering what risks are associated with a particular hazard, including the mechanisms that cause the hazard, then estimating the likelihood that the hazard will occur and the consequences if it does occur.

Risk assessment is the determination of the acceptability of risk using two dimensions of measurement – the likelihood of an adverse event occurring; and the severity of the consequences if it does occur, as illustrated in the diagram below.  (This two-dimensional risk assessment is a conceptually useful way of ranking risks, even if one or both of the dimensions cannot be measured quantitatively).


By way of illustration, the likelihood of something bad happening could be very low, but the consequences could be unacceptably high – enough to justify preventative action.  Conversely, the likelihood of an event could be higher, but the consequences could low enough to justify ‘taking the risk’.

In assessing the consequences, consideration needs to be given to the size of the population likely to be affected, and the severity of the impact on those affected.  This will provide an indication of the aggregate effect of an adverse event.  For example, ‘high’ consequences might include significant harm to a small group of affected individuals, or moderate harm to a large number of individuals.

A fallacy is committed when a person either focuses on the risks of an activity and ignores its benefits; and/or takes account one dimension of risk assessment and overlooks the other dimension.

To give a practical example of a one-dimensional risk assessment, the desalination plant to augment Melbourne’s water supply has been called a ‘white elephant’ by some people, because it has not been needed since the last drought broke in March 2010.  But this criticism ignores the catastrophic consequences that could have occurred had the drought not broken.  In June 2009, Melbourne’s water storages fell to 25.5% of capacity, the lowest level since the huge Thomson Dam began filling in 1984.  This downward trend could have continued at that time, and could well be repeated during the inevitable next drought.


Melbourne’s desalination plant at Wonthaggi

No responsible government could afford to ‘take the risk’ of a major city of more than four million people running out of water.  People in temperate climates can survive without electricity or gas, but are likely to die of thirst in less than a week without water, not to mention the hygiene crisis that would occur without washing or toilet flushing.  The failure to safeguard the water supply of a major city is one of the most serious derelictions of government responsibility imaginable.

Turning now to the anti-vaccination and anti-fluoridation movements, they both commit the fallacy of Faulty Risk Assessment.  They focus on the very tiny likelihood of adverse side effects without considering the major benefits to public health from vaccination and the fluoridation of public water supplies, and the potentially severe consequences of not vaccinating or fluoridating.

Vaccination risks

The benefits of vaccination far outweigh its risks for all of the diseases where vaccines are available.  This includes influenza, pertussis (whooping cough), measles and tetanus – not to mention the terrible diseases that vaccination has eradicated from Australia such as smallpox, polio, diphtheria and tuberculosis.

As fellow skeptic Dr. Rachael Dunlop puts it:  ‘In many ways, vaccines are a victim of their own success, leading us to forget just how debilitating preventable diseases can be – not seeing kids in calipers or hospital wards full of iron lungs means we forget just how serious these diseases can be.’

No adult or teenager has ever died or become seriously ill in Australia from the side effects of vaccination; yet large numbers of people have died from the lack of vaccination.  The notorious Wakefield allegation in 1998 of a link between vaccination and autism has been discredited, retracted and found to be fraudulent.  Further evidence comes from a recently published exhaustive review examining 12,000 research articles covering eight different vaccines which also concluded there is no link between vaccines and autism.

According to Professor C Raina MacIntyre of UNSW, ‘Influenza virus is a serious infection, which causes 1,500 to 3,500 deaths in Australia each year.  Death occurs from direct viral effects (such as viral pneumonia) or from complications such as bacterial pneumonia and other secondary bacterial infections. In people with underlying coronary artery disease, influenza may also precipitate heart attacks, which flu vaccine may prevent.’

In 2010, increased rates of high fever and febrile convulsions were reported in children under 5 years of age after they were vaccinated with the Fluvax vaccine.  This vaccine has not been registered for use in this age group since late 2010 and therefore should not be given to children under 5 years of age. The available data indicate that there is a very low risk of fever, which is usually mild and transient, following vaccination with the other vaccine brands.  Any of these other vaccines can be used in children aged 6 months and older.

Australia was declared measles-free in 2005 by the World Health Organization (WHO) – before we stopped being so vigilant about vaccinating and outbreaks began to reappear.  The impact of vaccine complacency can be observed in the 2015 measles epidemic in Wales where there were over 800 cases and one death, and many people presenting were of the age who missed out on MMR vaccination following the Wakefield scare.

After the link to autism was disproven, many anti-vaxers shifted the blame to thiomersal, a mercury-containing component of relatively low toxicity to humans.  Small amounts of thiomersal were used as a preservative in some vaccines, but not the MMR vaccine.  Thiomersal was removed from all scheduled childhood vaccines in 2000.

In terms of risk assessment, Dr. Dunlop has pointed out that no vaccine is 100% effective and vaccines are not an absolute guarantee against infection. So while it’s still possible to get the disease you’ve been vaccinated against, disease severity and duration will be reduced.  Those who are vaccinated have fewer complications than people who aren’t.  With pertussis (whooping cough), for example, severe complications such as pneumonia and encephalitis (brain inflammation) occur almost exclusively in the unvaccinated.  So since the majority of the population is vaccinated, it follows that most people who get a particular disease will be vaccinated, but critically, they will suffer fewer complications and long-term effects than those who are completely unprotected.

Fluoridation risks

Public water fluoridation is the adjustment of the natural levels of fluoride in drinking water to a level that helps protect teeth against decay.  In many (but not all) parts of Australia, reticulated drinking water has been fluoridated since the early 1960s.

The benefits of fluoridation are well documented.  In November 2007, the NHMRC completed a review of the latest scientific evidence in relation to fluoride and health.  Based on this review, the NHMRC recommended community water fluoridation programs as the most effective and socially equitable community measure for protecting the population from tooth decay.  The scientific and medical support for the benefits of fluoridation certainly outweighs the claims of the vocal minority against it.

Fluoridation opponents over the years have claimed that putting fluoride in water causes health problems, is too expensive and is a form of mass medication.  Some conspiracy theorists go as far as to suggest that fluoridation is a communist plot to lower children’s IQ.  Yet, there is no evidence of any adverse health effects from the fluoridation of water at the recommended levels.  The only possible risk is from over-dosing water supplies as a result of automated equipment failure, but there is inline testing of fluoride levels with automated water shutoffs in the remote event of overdosing.  Any overdose would need to be massive to have any adverse effect on health.  The probability of such a massive overdose is extremely low.

Tooth decay remains a significant problem. In Victoria, for instance, more than 4,400 children under 10, including 197 two-year-olds and 828 four-year-olds, required general anaesthetic in hospital for the treatment of dental decay during 2009-10.  Indeed, 95% of all preventable dental admissions to hospital for children up to nine years old in Victoria are due to dental decay. Children under ten in non-optimally fluoridated areas are twice as likely to require a general anaesthetic for treatment of dental decay as children in optimally fluoridated areas.

As fellow skeptic and pain management specialist Dr. Michael Vagg has said, “The risks of general anaesthesia for multiple tooth extractions are not to be idly contemplated for children, and far outweigh the virtually non-existent risk from fluoridation.”  So in terms of risk assessment, the risks from not fluoridating water supplies are far greater than the risks of fluoridating.

Implications for skeptical activism

Anti-vaxers and anti-fluoridationists who are motivated by denialism and conspiracy theories tend to believe whatever they want to believe, and dogmatically so.  Thus evidence and arguments are unlikely to have much influence on them.

But not all anti-vaxxers and anti-fluoridationists fall into this category.  Some may have been misled by false information, and thus could possibly be open to persuasion if the correct information is provided.

Others might even be aware of the correct information, but are assessing the risks fallaciously in the ways I have described in this article.  Their errors are not ones of fact, but errors of reasoning.  They too might be open to persuasion if education about sound risk assessment is provided.

I hope that analysing the false beliefs about vaccination and fluoridation from the perspective of the Faulty Risk Assessment Fallacy has provided yet another weapon in the skeptical armoury against these false beliefs.


Rachael Dunlop (2015) Six myths about vaccination – and why they’re wrong. The Conversation, Parkville.

C Raina MacIntyre (2016) Thinking about getting the 2016 flu vaccine? Here’s what you need to know. The Conversation, Parkville.

Mike Morgan (2012) How fluoride in water helps prevent tooth decay.  The Conversation, Parkville.

Michael Vagg (2013) Fluoride conspiracies + activism = harm to children. The Conversation, Parkville.

 Government of Victoria (2014) Victorian Guide to Regulation. Department of Treasury and Finance, Melbourne.

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