Confusing correlation with causation: rooster syndrome

The rooster crows and the sun rises: cause and effect, or red flag?

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Climate scientists do not immediately jump to the conclusion, on seeing this graph, that the burning of fossil fuels is causing climate change. This conclusion has been reached only after much evidence has been gathered, and a mechanism identified. As James Lawrence Powell explains:

If we knew nothing but the chart, we would know only that fossil fuel emissions, atmospheric CO2, and temperature correlate. That would seem unlikely to represent coincidence, but we would not know which of the three factors might have caused the others. And, as always, we would need to remember that correlation is not causation. But in this case we do know more: we know that burning fossil fuels has put increasing amounts of CO2 into the atmosphere. We know from carbon isotope studies that the increased carbon dioxide in the atmosphere came from fossil fuel combustion rather than from some natural cause. We know that the greenhouse effect, a fact of physics, requires that the more CO2 in the atmosphere, the higher the temperature.
To take the opposite view and deny that the chart shows cause and effect requires one to believe two things for which there is no evidence: first, during the twentieth century, some unknown factor suppressed the greenhouse effect, preventing the increase in carbon dioxide from causing the increase in temperature. … Second, if we grant for the sake of argument that something did suppress the greenhouse effect, then some natural process must have caused the temperature increase. But … scientists have investigated each natural cause of temperature variation – the Sun’s activity, atmospheric ozone, volcanic eruptions, sulfur aerosols – and none by itself or in any combination can explain the twentieth century temperature increase.

• Global warming deniers often point out that, historically, atmospheric carbon dioxide has increased after global temperature rises, not before. They then argue that this proves carbon dioxide cannot cause global warming. In fact, the situation is a classic example of a self-reinforcing system. CO₂ causes temperature rises which in turn cause release of CO₂ to the atmosphere, and so on…. Past episodes of global warming have indeed mostly been triggered by increases in solar energy. The present episode of global warming has been sparked by increased atmospheric CO₂ from burning of fossil fuels. The warming is in turn releasing more CO₂, which is reinforcing the warming.
• Here’s an unfortunate example from my local newspaper. A veterinarian treats a traumatised kitten with conventional methods, then resorts to acupuncture. The cat survives, and she ascribes this to the acupuncture. Why wasn’t it the conventional treatment? And why wasn’t it simply a case of reversion to the mean – the animal would have recovered anyway? The only way to tell is to use randomised controlled trials, and these show little evidence that acupuncture is effective.
• Medical researchers from Glasgow University found a correlation between tea-drinking and prostate cancer. However, they did not assume this indicated a causal relationship. In the words of the lead researcher:

Most previous research has shown either no relationship with prostate cancer for black tea or some preventive effect of green tea. We don’t know whether tea itself is a risk factor or if tea drinkers are generally healthier and live to an older age when prostate cancer is more common anyway. We found that heavy tea drinkers were more likely not to be overweight, be non alcohol-drinkers and have healthy cholesterol levels. However, we did adjust for these differences in our analysis and still found that men who drank the most tea were at greater risk of prostate cancer.

• Homes with swimming pools use 49% more energy than homes without. The temptation is to blame the swimming pool for the extra energy. In fact, homes with pools are bigger and have more occupants. Those occupants have higher incomes and a lifestyle geared to high energy consumption. So while the pool accounts for some of the extra energy, it’s a symptom rather than a cause.
• Update 2014/02/12: Anti-vaccination campaigners vigorously attempt to prove a connection between vaccinations and autism, without any success (because there is none). This graph shows how easy it is to “prove” autism is caused by organic food if you surrender to rooster syndrome.
  
• Update 2014/05/11: For a selection of graphs that emphatically illustrate the absurdity of rooster syndrome, visit the Spurious Correlations website.

Further reading

Clearing up confusion between correlation and causation at The Conversation.

After This, Therefore Because of This: Your Weird Relationship with Cause and Effect podcast at YANSS.

Add to the list of examples by leaving a comment.