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This article, now with additions and amendments, was first published in volume 14 of the 'Skeptical Intelligencer', 2011, pp.10-14.

Recently there was much interest shown by the media in an investigation reported in Nature which concluded that 'intrinsic risk factors contribute only modestly (less than 10-30 per cent of lifetime risk) to cancer development'. That is, most cases of cancer result from avoidable factors such as toxic chemicals and radiation (note 1).

One reason for the attention given to this research was that earlier this year a cancer study was widely reported that concluded, 'Only a third of the variation in cancer risk among tissues is attributable to environmental factors or inherited predispositions. The majority is due to "bad luck", that is, random mutations arising during DNA replication in normal, noncancerous stem cells' (note 2).

The results of the two studies appear to be contradictory. Perhaps in the fullness of time this contradiction will be resolved but for the moment the lay public will once again be asking, 'What are we supposed to believe?' and bemoaning the fact that hardly a week passes when the media announce that scientists have made a discovery that contradicts previous research findings (which may also have contradicted research findings prior to these). Inevitably people ask, 'Can we really trust science?'

Any research that involves human beings is particularly liable to a confusion of evidence. This is not just the case for the social sciences but also biological sciences (human physiology, biochemistry, genetics, medicine, etc.). Perhaps the main reason is that human beings are so complex. In fact, an attitude of skepticism to the announcement of any new scientific finding or discovery is in itself good scientific practice (hence we may say that science answers questions but also question answers). This is certainly the case with any observations or conclusions that are not immediately predictable from current scientific knowledge.

The rules of science are simple in theory but deceptively difficult in practice because so much else intrudes into the thought process - notably cognitive biases of various kinds. The major rule is that the preferred explanation is the one that is most consistent with all available knowledge. So when people sincerely report that during the night they were visited by strange beings, raised out of their bed and onto some kind of spacecraft, were subjected to a surgical operation, etc. and then returned to their bed, we stick with explanations that fit best with what we already know about the world. One such explanation in this case is that the person was dreaming or experiencing sleep paralysis rather than being temporarily abducted by extraterrestrials.

Our adopting the most likely explanation is not the same as saying that the unusual explanation is wrong, and, who knows, by applying this rule rigidly scientists may be missing something very important - alien beings might be visiting planet Earth. But there is a very good reason for applying this rule. If we adopt the unusual explanation, we are left with many more things to explain than if we go for the most likely one. This is the opposite of the scientific method. In the present case we have to account for how the extraterrestrials managed to make a journey of trillions of miles; why their craft was not spotted by radar defences; why the neighbours didn't hear what was happening; how the surgical operation was accomplished with no evidence of scarring; etc., etc.

So the rule is if, based on our existing knowledge of the world, there is at least one explanation more likely than the one being offered, don't accept the latter. This precept has a long history. The 18th century Scottish philosopher David Hume pointed out that we should only agree that a miraculous explanation is true when it would be even more miraculous for it not to be true (from which it would follow that all other available explanations, such as mistaken or false reporting, would be even more miraculous). So supposing you showed me a photograph purporting to be of a prehistoric monster in Loch Ness monster and I ask you, 'Is it possible that there could be a more mundane explanation for this photo, for example that it is a fake?' If you reply, 'Impossible! I know the person who took this photo and he is as honest as the day is long' then I'm sorry but I will not accept your 'miraculous' explanation. Notice that I do not have to say to you, 'It must be a fake' because then it is up to me to show you that it is. Maybe you are right and it is a prehistoric monster, but so long as there is a possible explanation that is consistent with what we know about the world then I (being a good scientist I hope) will not accept an explanation that is not so.

Incidentally, this is why so many scientific experiments that claim to have demonstrated the authenticity of paranormal phenomena such as ESP (extra-sensory perception) have been treated with such skepticism. It is absolutely essential that in this kind of research, the experimental conditions are completely watertight so any target information could not have been communicated in any other way. So suppose I tell you that I have just done an experiment on telepathy and obtained highly significant results. You ask me, 'Was there ever a time when you left the participants alone in the room with the test materials?' 'Well, er … yes', I reply, 'but none of them would cheat - they're not like that!'. You must then say to me, 'Sorry. I can't accept a paranormal explanation for your results (and not 'Your results were due to cheating by the participants', a serious allegation that you then have to defend).

Of course, this doesn't prove that the material was not communicated by telepathy, but the onus is on me, the experimenter, to eliminate this 'more likely' explanation - and others - by further research with improved experimental procedures.

This rule holds not just for the case where a paranormal explanation is being considered; it applies throughout science (and in other scholarly disciplines such as history and indeed in everyday life) when new findings or discoveries require an unusual or far-reaching explanation. For example, in October 2015 I was thrilled to hear that a researcher at the California Institute of Technology, Ranga-Ram Chary, reported that while mapping the cosmic background radiation he detected an unusual glow in one area (note 3). Chary believed that this could be evidence of the existence of other universes (the multiverse hypothesis). He suggested that this radiation could be 'due to the collision of our Universe with an alternate Universe whose baryon-to-photon ratio is a factor of around 65 larger than ours'. However, theoretical astrophysicist David Spergel from Princeton University thinks it is worth looking into explanations that do not involve other universes, such as dust. 'The dust properties are more complicated than we have been assuming, and I think that this is a more plausible explanation' he says (note 4). So for the time being it is good science not to accept the multiverse explanation.

In this way science reflects on itself skeptically. And in another way: when there are observations or discoveries that, if correct, have significant consequences for our understanding of the world, the reaction should be to wait until the results have been replicated a sufficient number of times, maybe with adjustments to eliminate competing explanations (notes 5 and 6).

Scientific explanations can only be based on the evidence available at the present time. Tomorrow, new reliable evidence may emerge that is not consistent with existing explanations. These explanations must then be modified or rejected in favour of ones that can account for the new findings. But any new explanation must be consistent with not merely the new evidence, but also the evidence that existed before the new discovery. For example, Isaac Newton's theory of gravity was brilliantly successful in explaining and predicting the movement of objects large and small, but it was eventually shown to be inaccurate under certain extreme circumstances. Einstein's general theory of relativity is now the preferred account of gravitational attraction, though calculations based on Newton's formulation can still be relied on for most purposes.

In reality, any theory may not be able to explain all the evidence. Scientists have to keep trying to tweak their theories so that they account for any anomalous findings. But as our knowledge of the world increases, it becomes increasingly less likely that the important and wide-ranging theories of, say, our universe and the evolution of life will eventually be rejected wholesale.

So the public need not perceive a lengthy timeline of contradictory scientific findings and theories as evidence that there is something at fault with the scientific method (though scientists themselves are certainly fallible). It is the nature of scientific enquiry that this must be so. We do our best to construct the material world accurately through our senses, but to arrive at real, reliable truths about that world is more difficult than we imagine. 'We see through a glass darkly', as St Paul said. And science, skeptically applied, is the best means we have of penetrating this darkness.


  1. Wu S., Powers S., Zhu W. & Hannun Y.A. (2015) 'Substantial contribution of extrinsic risk factors to cancer development', Nature doi:10.1038/nature16166.
  2. Tomasetti C. & Vogelstein, B (2015) Variation in cancer risk among tissues can be explained by the number of stem cell divisions. Science, 347, 78-81.
  3. R. Chary (2015) 'Spectral variations of the sky: Constraints on alternate universes'
  5. Actually, failure to replicate positive results has also bedevilled laboratory research into paranormal claims such as telepathy.
  6. For this reason, amongst others, accounts of one-off observations such as UFO sightings, ghosts and strange coincidences are not usually good material for scientific investigation.