Saturday, November 19, 2011

Weekend Post - What do modern medicine, flu, ice cream and pirates have in common?

Here are some dangerous but factual observations.

Since the introduction of modern healthcare in Botswana more people than ever before have died of cancer. People suffer from more infectious diseases in colder seasons. The more ice cream that is consumed, the higher the number of drownings that occur. The reduction in the number of pirates (yes, I said pirates) in the world has coincided with gradually increasing global temperatures.

All of these are facts, there is overwhelming evidence to support them. But the big question is what they actually mean. Is it true that modern healthcare causes cancer? Does cold weather cause colds and flu? Does ice cream cause people to drown? Has the loss of pirates actually caused global warming? Should any of us actually do anything, should we take any lifestyle decisions based on these undeniable facts?

Of course not, but that’s where the danger lies. You only have to hear of these facts for a chill to go through you. It’s only one step further before you begin to suspect the benefits of modern medicine and science.

The real facts are simple. More people are dying of cancer since the introduction of modern, scientifically based medicine because we’re living longer. Cancer is largely a disease of older people. As we’ve done away with so many of the causes of death from the past that killed younger people like infectious disease and starvation other causes of death have stepped in to fill the void. Like cancer.

People suffer more colds and flu in the winter, not because our immune systems are weaker in the cold months or because viruses are stronger but simply because when we’re cold we stay inside much more. When we’re stuck inside our homes with our families we’re much more likely to spread infections by close-proximity coughs and sneezes.

The really good news is that ice cream doesn’t make you more likely to drown. However, both rates of drowning and consumption of ice cream are both associated with hot weather. Both are caused by something else.

One of the easiest mistakes to make when looking at the world is to assume that correlation implies causation. Because two observations are somehow related then one must have caused the other. It’s a fundamental mistake and one of many logical fallacies that people often make.

A recent example. A couple of weeks ago the BBC reported that scientists had announced in the British Medical Journal “a link” between use of the oral contraceptive pill and prostate cancer. The interesting thing is that only women take the pill and only men get prostate cancer so how can one possibly cause the other? The suggestion is that the hormone oestrogen in the contraceptive pill may be released in the urine of pill-taking women into the water supply. Men then drink the water and experience one of the side-effects of oestrogen: higher rates of cancer. The team of researchers from Canada had carefully examined various countries that used of different forms of contraception and found higher levels of prostate cancer in those countries where the pill was widely used. In countries where other forms of contraception were widespread the higher levels of cancer couldn’t be found.

However they were very careful to say that their study “has significant limitations with respect to causal inference”. In other words they are most certainly NOT saying that the pill increases the risk of prostate cancer. This was just what they call “an ecological study”. They observed a correlation between two things and said that “it must be considered hypothesis generating, and thought provoking”. This wasn’t an experiment, it was just some careful observations that make scientists want to ponder a bit and then design more detailed studies.

One problem is that it’s very difficult to design an experiment to test this. If we suspect there’s a link between oestrogen and prostate cancer we can’t just expose a group of men to oestrogen and see if they develop cancer, that would be inhumane. However it would be worth conducting animal tests and further ecological studies. For instance, does the rate of prostate cancer go down in countries where the use of the pill declines or where lower oestrogen pills are used?

Finally, in case you were worried, nobody really believes there’s a relationship between the number of pirates and global warming. However if you’re willing to suspend your skeptical faculties you can join the Church of the Flying Spaghetti Monster for whom this is a core belief. Perhaps not entirely seriously!


For various sources on "Correlation Does Not Imply Causation" see the Skeptics Dictionary chapter here (180kb pdf download) or the Wikipedia page here. There's also a comprehensive list of the many logical fallacies courtesy of the Skeptic's Guide to the Universe here.

The story from the BBC about the pill and prostate cancer can be seen here and the original article in the British Medical Journal here.

For the Church of the Flying Spaghetti Monster just say "Arrrgh".

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