How Do We Know What’s Responsible for Illness?

Dr Kathryn Fox is an Australian medical doctor and bestselling crime writer, best known for her forensic thrillers featuring pathologist Dr Anya Crichton. Drawing on her medical expertise, she crafts gripping, authentic crime fiction and is also a passionate advocate for forensic medicine education and public engagement. 

One of the most common questions doctors are asked sounds deceptively simple.

“What caused it?”

Sometimes patients are asking about a rash.

Sometimes a cancer diagnosis.

Sometimes a chronic illness that had appeared without warning.

Although it may sound like a simple question, it’s one of the most difficult questions in medicine.

A broken wrist after a fall on an outstretched hand is clear.

“I’ve been getting headaches since I bought a new phone,” makes sense that the two could be related. But it doesn’t prove the phone is the cause of the headaches. It could just be a coincidence.

“I’ve been getting headaches since I started a new job,” is still a connection but there are so many questions that need to be answered before determining the cause or concluding the job is responsible.

Human beings are natural pattern-seekers. We constantly look for explanations. When two events are close together, our brains instinctively link them. And if there seems to be a possible biological link, we quickly latch on to that notion.

Something happened. Then something else happened.

It seems to make sense that the first thing caused the second.

Noticing a pattern is important. In fact, that’s often how discoveries start. But scientists then need to work out whether the pattern is real, whether it is coincidence, or whether something else entirely is responsible.

Smoking and lung cancer are a good example of the process in determining cause and effect.

For years smoking was promoted as ‘healthy’. Doctors proudly advertised it, cigarette companies sponsored sports like cricket and rugby league. I even remember a cardiologist who smoked during his consults with patients. Back then, smoking was assumed to be part of a healthy lifestyle.

Then researchers noticed that smokers developed lung cancer more frequently than non-smokers. There was a correlation between smoking and cancer.

Correlation is where science begins.

So did smoking cause cancer?

Causation is where science gets difficult.

Scientists looked deeper.

Did cigarette smoke contain substances capable of causing cancer?

Yes.

Did heavy smokers have higher cancer rates than light smokers?

Yes.

Did the relationship appear repeatedly in different countries and populations?

Yes.

Did lung cancer rates eventually fall as smoking rates fell?

Again, yes.

Gradually, the evidence became overwhelming. Not because of one study, but because a plethora of peer-reviewed evidence with large sample sizes all pointed in the same direction.

That’s how medicine builds a case.

A bit like a detective solving a crime.

One clue or study is never enough.

You need multiple lines of evidence before you can confidently identify the culprit or exonerate one.

When I was a child, tattoos were rare. Women painted their own nails at home. There were no mobile phones, no social media, no Wi-Fi. We rarely ate processed food and fast-food outlets were few and far between. At the same time, autism rates were much lower than today.

Can we conclude that any of these are responsible for the rise in autism? No. All we can say is that there is a correlation between rises in certain conditions. Without extensive evidence, we can’t draw any conclusions. And so many factors and variables need to be considered.

Over the same period, plenty of things have decreased as well.

Asbestos use has fallen dramatically. Leaded petrol disappeared. Smoking rates declined.

But we don’t assume that asbestos, lead exposure and cigarette smoke protected against autism.

The public often wants certainty. It makes decisions about our health easier.

Medicine is usually dealing with probabilities.

Most diseases don’t have a single cause. Genetics, environment, lifestyle and chance often interact in complicated ways.

That’s why the question “What caused it?” is often far harder to answer than people imagine.

And why doctors are cautious about drawing conclusions simply because two things happened at the same time.

Because if medicine has taught me anything, it’s that coincidence can be very convincing.

Some of the most obvious explanations turn out to be wrong.

A last thought. I was in an Emergency Department during the very first State of Origin match. For the first time ever, not one patient came in once the game started. We all excitedly hoped that Rugby League would be the cure for the chaos in emergency.

And there was a flow-on effect. There were no car accidents with so few cars on the road. No overdoses and no Maybe this was the public health initiative we all needed.

Then the half-time whistle blew, and the ambulances rolled in, many containing men with chest pain. In the space of two hours, the State of Origin seemed responsible for saving lives AND causing heart attacks. I dare anyone to draw a scientific conclusion from that.