本文作者：小红猪小分队

【In the library, books about physics endure longer than those on psychology (Image: PlainPicture)】

Much of what we believe to be factual has an expiration date, but the good news is that we can see it coming

Read more: “Handle the truth: Navigating knowledge in the web age“

IN DENTAL school, my grandfather was taught the number of chromosomes in a human cell. But there was a problem.

Biologists had visualised the nuclei of human cells in 1912 and counted 48 chromosomes, and it was duly entered into the textbooks studied by my grandfather. In 1953, the prominent cell biologist Leo Sachs even said that “the diploid chromosome number of 48 in man can now be considered as an established fact”.

Then in 1956, Joe Hin Tjio and Albert Levan tried a new technique for looking at cells. They counted over and over until they were certain they could not be wrong. When they announced their result, other researchers remarked that they had counted the same, but figured they must have made a mistake. Tjio and Levan had counted only 46 chromosomes, and they were right.

Science has always been about getting closer to the truth, and anybody who understands it knows that a continual transformation of accepted knowledge along the way is how it works. However, sometimes it can feel random and unsettling. Smoking has gone from doctor-recommended to deadly. Eating meat used to be good for you, then bad, then good again; now it’s a matter of opinion. The age at which women are told to get mammograms has risen. We used to think that Earth was the centre of the universe, and our planet has since been demoted. I have no idea any longer whether or not red wine is good for me.

It turns out that there is order within the shifting noise. The good news emerging from my field of scientometrics – the quantitative study of science – is that in the aggregate there are regularities to the changes, and we can even identify how fast facts will decay over time. This means we don’t have to be all at sea in a world of changing knowledge. Portions of what we assume to be true will eventually be overturned, but realising that patterns exist could help to identify the apparent truths that are poised to expire.

It is obvious that scientific knowledge is continually updated through new discoveries and the replication of studies, but until recent years little attention had been paid to how fast this change occurs. In particular, few had attempted to quantify how long it would take what we know at any given moment to become untrue, or replaced with a closer approximation of the truth.

Among the first groups to measure this churning of knowledge was a team of researchers at Pitié-Salpêtrière hospital in Paris, France. To get a handle on it, Thierry Poynard and his colleagues chose to focus on medical fields in which they specialised: cirrhosis and hepatitis, two areas related to liver diseases. They took nearly 500 articles in these fields from over 50 years and gave them to a panel of experts to examine. Each expert was charged with saying whether the paper was factual, out-of-date or disproved (Annals of Internal Medicine, vol 136, p 888).

Through doing this, Poynard and his colleagues were able to create a simple chart that showed the amount of factual content that had persisted over the previous decades (see diagram). They found something striking: a clear decay in the number of papers that were still valid. Furthermore, it was possible to get a clear measurement for the “half-life” of facts in these fields by looking at where the curve crosses 50 per cent on this chart: 45 years.

Essentially, information can be likened to a radioactive material. Medical knowledge about cirrhosis or hepatitis takes about 45 years for half of it to become out of date or disproved.

The half-life metaphor does not match up exactly to its radioactive namesake. For one thing, the time frames being examined make it difficult to see if the decay is truly exponential. Similarly, depending on the maturity of the field, the half-life need not be constant. Indeed, it has surely changed as medicine transitioned from art to science. Nonetheless, half-lives can be a useful way of thinking about the decay of knowledge.

We can’t predict which individual papers will be overturned, of course, just like we can’t tell when individual radioactive atoms will decay, but we can observe the aggregate and see that there are rules for how a field changes over time. The cirrhosis and hepatitis results were nearly identical to an earlier study that examined the overturning of information in surgery. Two Australian surgeons found that half of the facts in that field also become false every 45 years (The Lancet, vol 350, p 1752)

Unfortunately, convening a panel of experts to comb through all of science’s past conclusions isn’t feasible. So we have to sacrifice precision for our ability to look at lots of science relatively quickly. One simpler way to do this is by looking at the lifetime of citations, the coin of the scientific realm and the metric by which the impact of a research paper is measured.

To understand the decay in the truth of a paper, we can measure how long it takes for people to stop citing the average paper in a field. Whether it is no longer interesting, no longer relevant or has been contradicted by new research, this paper is no longer a part of the living scientific literature. The amount of time it takes for others to stop citing half of the literature in a field is also a half-life of sorts.

Through this we can begin to get rough estimates of the half-lives of many fields. For example, a study of all the papers in the Physical Review journals, a cluster of periodicals of great importance to physicists, found that the half-life in physics is about 10 years (arxiv.org/abs/physics/0407137).

Different publication formats can also have varied half-lives. In 2008, Rong Tang of Simmons College in Boston looked at scholarly books in different fields and found that physics has a longer half-life (13.7 years) than economics (9.4), which in turn outstays mathematics, psychology and history (College & Research Libraries, vol 69, p 356).

This is the opposite of what is found in journal articles, where knowledge at the frontiers of the hard sciences is overturned more rapidly than in the social sciences. This may be because immediate repeated experimentation can be more straightforward in the physical sciences than in the social sciences, where the data can be noisier.

So drawing strong conclusions about the differences between the calculated half-lives of disparate fields should be approached with caution, because of the various influencing factors and different methods employed to gauge their longevity. Still, the point remains that specific bodies of knowledge can be viewed as having different expiry dates. This leads one to wonder whether reframing knowledge in this way should change how we interpret the facts that each of us use to navigate the world. After all, identifying that certain bodies of facts will endure longer than others might influence how we act on information – and identify what facts we should be most wary of.

The spectrum of truth

Let’s imagine that we were to line up facts on a spectrum according to how quickly they change. On the far left, we have the groups of fast-changing facts, which are constantly in flux. These would be our knowledge about what level the stock markets closed at yesterday, or predictions for weather around the globe. You might say that these areas have tiny half-lives: the length of time they stay true is very brief.

On the far right, we have the very slow-changing knowledge, featuring the facts that for all practical purposes are constant. Individually, this might include what we’ve learned about the number of continents on the planet, and on a group level, you might place pretty much everything the ancient Greeks wrote about geometry.

In between, we would have the groups of facts that change, but not too quickly. This knowledge might change over the course of years or decades or a single lifetime. These facts in the middle are what I refer to as “mesofacts” – and their relatively slow rate of change may mean that many people fail to acknowledge their ultimate transience and looming expiry.

Knowledge about nutrition might be considered to be one of these mesofactual areas. For example, in the US, we no longer use the four basic food groups – meats, dairy, cereals/breads, fruits/vegetables. There are now five food groups placed on a plate. Between these points there was a food pyramid that underwent multiple overhauls. We have also experienced long-term changes with regard to whether we should eat fatty foods, carbohydrates and many other things.

Another mesofactual realm might be how we are supposed to take care of babies. Each generation has a set of facts that changes, from whether babies should sleep on their backs or stomachs to whether pregnant women can safely smoke or drink alcohol.

Mesofacts are all around us, and just acknowledging their existence can be useful. After all, as studies of our perception of slow shifts in the natural world have revealed, we are often blind to gradual change (see “Too slow to see“).

Of course, this doesn’t mean that everything is going to be overturned. If a government-funded study promotes a certain health practice, for example, we should not immediately dismiss it as being based on one of many evanescent facts. That would be foolishly throwing the informational baby out with the bath water. But we should not be shocked if that advice is later contravened.

Fortunately, we are getting better at internalising this particular truth: many medical schools inform their students that within five years, half of what they have been taught will be wrong – and the teachers don’t know which half it will be.

Thankfully, facts don’t expire arbitrarily. Even though knowledge changes, the astounding thing is that it changes in a regular manner. If we recognise this, we’ll be better prepared to live in the rapidly changing world around us.

Too slow to see

When British economist John Maynard Keynes was asked why he switched his position on monetary policy, he uttered the immortal, and possibly apocryphal, bon mot: “When the facts change, I change my mind. What do you do, sir?” All too often we don’t act like Keynes, especially when the facts change slowly.

One particular failure of human perception is known as shifting baseline syndrome. The term was coined by marine biologist Daniel Pauly of the University of British Columbia in Vancouver, Canada, to refer to what happened with fish populations. When Europeans began fishing off Newfoundland and Cape Cod in the 17th century, fish were incredibly abundant, but in less than 200 years, many species were entirely wiped out. How could this have been allowed to happen? Pauly argued that it was because each generation of fisheries’ scientists accepts as a baseline the stock size and species composition in place at the beginning of their careers, and uses this to evaluate changes. The result is a gradual shift of the baseline, accommodating the creeping disappearance between generations.

The syndrome has since been extended beyond human generations to lifetimes – in particular, the way that people are blind to slow changes in their environment over many years. For example, in 2009, researchers surveyed people living in northern England about their perceptions of local bird populations during the previous two decades. They found that more than a third had completely failed to notice that the most common species had changed (Conservation Letters, vol 2, p 93).

The message is that we may be continually setting new baselines in our personal knowledge throughout our lifetimes. This failure to spot gradual change can result in disaster, ecological or otherwise.

Extinct in a blink

It’s easy to mistakenly assume that some of the facts in our heads are absolute, especially those learned in the textbooks of our youth (see main story). As a child, I loved learning about dinosaurs. But I have since discovered an incorrect fact that I had lived my childhood assuming was accurate: the name Brontosaurus.

The four-legged saurischian, with its long neck and tiny head, is iconic. And yet its name is actually Apatosaurus. Why? In 1978, two palaeontologists noticed that the skeleton used to identify the Brontosaurus species had been graced with the skull of a different plant-eating dinosaur. The body belonged to the Apatosaurus. The Brontosaurus never existed.

Since then, scientists have promoted the name change, and it has gained some currency. Nevertheless, the Brontosaurus myth continues to endure in popular knowledge and books – no doubt aided by people who missed the expiry of this fact.