Correlation is not a cause

The phrase "correlation is not a cause" (CINAC) may be familiar to every scientist but has not found its way into everyday language, even though critical thinking and scientific understanding would improve if more people had this simple reminder in their mental toolkit.

One reason for this lack is that CINAC can be surprisingly difficult to grasp. I learned just how difficult when teaching experimental design to nurses, physiotherapists and other assorted groups. They usually understood my favourite example: imagine you are watching at a railway station. More and more people arrive until the platform is crowded, and then — hey presto — along comes a train. Did the people cause the train to arrive (A causes B)? Did the train cause the people to arrive (B causes A)? No, they both depended on a railway timetable (C caused both A and B).

I soon discovered that this understanding tended to slip away again and again, until I began a new regime, and started every lecture with an invented example to get them thinking.

"Right", I might say "Suppose it's been discovered (I don't mean it's true) that children who eat more tomato ketchup do worse in their exams. Why could this be?" They would argue that it wasn't true (I'd explain the point of thought experiments again). "But there'd be health warnings on ketchup if it's poisonous" (Just pretend it's true for now please) and then they'd start using their imaginations.

"There's something in the ketchup that slows down nerves", "Eating ketchup makes you watch more telly instead of doing your homework", "Eating more ketchup means eating more chips and that makes you fat and lazy". Yes, yes, probably wrong but great examples of A causes B — go on. And so to "Stupid people have different taste buds and don't like ketchup", "Maybe if you don't pass your exams your Mum gives you ketchup". And finally " "Poorer people eat more junk food and do less well at school".

Next week: "Suppose we find that the more often people consult astrologers or psychics the longer they live." "But it can't be true — astrology's bunkum" (Sigh … just pretend it's true for now please.) OK. "Astrologers have a special psychic energy that they radiate to their clients", "Knowing the future means you can avoid dying", "Understanding your horoscope makes you happier and healthier" Yes, yes, excellent ideas, go on. "The older people get the more often they go to psychics", "Being healthy makes you more spiritual and so you seek out spiritual guidance". Yes, yes, keep going, all testable ideas, and finally "Women go to psychics more often and also live longer than men."

The point is that once you greet any new correlation with "CINAC" your imagination is let loose. Once you listen to every new science story Cinacally (which conveniently sounds like "cynically") you find yourself thinking: OK, if A doesn't cause B, could B cause A? Could something else cause them both or could they both be the same thing even though they don't appear to be? What's going on? Can I imagine other possibilities? Could I test them? Could I find out which is true? Then you can be critical of the science stories you hear. Then you are thinking like a scientist.

Stories of health scares and psychic claims may get people's attention but understanding that a correlation is not a cause could raise levels of debate over some of today's most pressing scientific issues. For example, we know that global temperature rise correlates with increasing levels of atmospheric carbon dioxide but why? Thinking Cinacally means asking which variable causes which or whether something else causes both, with important consequences for social action and the future of life on earth.

Some say that the greatest mystery facing science is the nature of consciousness. We seem to be independent selves having consciousness and free will, and yet the more we understand how the brain works, the less room there seems to be for consciousness to do anything. A popular way of trying to solve the mystery is the hunt for the "neural correlates of consciousness". For example, we know that brain activity in parts of the motor cortex and frontal lobes correlates with conscious decisions to act. But do our conscious decisions cause the brain activity, does the brain activity cause our decisions, or are both caused by something else?

The fourth possibility is that brain activity and conscious experiences are really the same thing, just as light turned out not to be caused by electromagnetic radiation but to be electromagnetic radiation, or heat turned out to be the movement of molecules in a fluid. At the moment we have no inkling of how consciousness could be brain activity but my guess is that it will turn out that way. Once we clear away some of our delusions about the nature of our own minds, we may finally see why there is no deep mystery and our conscious experiences simply are what is going on inside our brains. If this is right then there are no neural correlates of consciousness. But whether it is or not, remembering CINAC and working slowly from correlations to causes is likely to be how this mystery is finally solved.