Tuesday, 5 February 2008

Presumption of innocence

I once heard a joke about a sheriff who stopped a woman in a car and cautioned her for illegally fishing, because her husband had left his fishing gear in the boot of the car and she was near a river. She was adamant she wasn't intending to fish, but the sheriff replied - "you've got the tackle - how do I know you're not going to use it?" In that case, parried the women, I'm accusing you of rape. The sheriff is gobsmacked - rape?! But he hasn't even touched her!! "I know," replies the women, "but you've got all the tackle - how do I know you're not going to use it?"

In English law we have the concept of the presumption of innocence - more commonly known by the phrase "innocent until proven guilty". The idea is that it's much harder to prove innocence than to prove guilt. If you are guilty the prosecution should be able to find witnesses to your crime, traces of your DNA or fingerprints at the crime scene. If you were home alone last night, no witnesses, no alibi, can you positively prove beyond doubt you were there and not up to no good elsewhere?

Sometimes guilt is easily proved. You were seen by witnesses, you left fingerprints, there are recorded phone conversations or CCTV images. Similarly cause and effect can be easy to demonstrate. Yesterday I walked into the open drawer of my desk (ouch) and today I have a big bruise on my shin. Cause and effect.

It's a little harder with health issues. It took years to link smoking and lung cancer. But it's not possible to say that if you smoke you will die of lung cancer - or that if you don't smoke you wont. Roy Castle, who never smoked, died from lung cancer. Conversely we all know elderly relatives who smoked 40 a day for 60 years or more and died peacefully in their beds of old age.

At the moment scientists keep looking at mobile phone use. So far there is no positive link between phone use and cancer - but is that because there is no link to find? Or because we haven't yet proved the link? Or haven't waited long enough for the cancers to start appearing?

What about aluminium and breast cancer? Various eminent scientists have said that there is no link, although they admit that more research needs to be done. Just because the police haven't yet had time to investigate a crime does it mean that crime hasn't been committed?

The debate that still rumbles on is MMR. MMR stands accused of causing autism. There is plenty of evidence - eyewitness accounts from hundreds of parents who had happy, normal children who were changed into troublesome and troubled children about the time of their MMR vaccine. The judge says that this evidence is circumstantial. Certainly MMR was hanging around looking suspicious - perhaps wearing a hoody and smoking - but that isn't proof that it committed the crime.

Since then their have been endless papers trying to prove MMR's innocence beyond doubt. Today MMR is in the papers again with yet another study failing to prove a link between MMR and autism. Is there no link? Or are scientists asking the wrong questions, looking at the wrong children, or the wrong numbers? (The Guardian calls this latest study "huge" but it only covers 250 children).

The key issues here are risk and evidence.

What is the actual risk of a child developing autism? Of an unvaccinated child catching measles? Of a child dying from measles? How can we understand relative risks? Read Reckoning with Risk, Risk by Lupton and Risk Matters in Healthcare.

And evidence - what is evidence? Are all health interventions guilty until proven innocent? Is it wise to assume every new idea will kill us until we can positively prove otherwise? Is evidence only evidence when it tells us what we want to hear? What value does the experience of ordinary people have in the hierarchy of evidence? If one scientist publishes a paper that says magnets don't relieve arthritis pain, but I can find 50 friends who say they do - who is right? What is good evidence? There are many books in the library on evidence, statistics, reading and understanding papers.

What can any of us do? Be vigilant. Be critical of the evidence. Use our brains.

No comments: