The Half-Life of Facts (5 page)

However it happens, scientific discovery marches forward. We are in an exceptional time, when the number of scientists is growing rapidly and consists of the majority of scientists who have ever lived. We have massive collaborative projects, from the Manhattan Project to particle accelerators, that have and are unearthing secrets of our cosmos. Yet, while this era of big science has allowed for the shockingly fast accumulation of knowledge, this growth of science is not unexpected. Both the growth of scientific facts itself as well as what allows discovery and innovation to plow ahead can be explained by scientometrics.

However, just as scientometrics can explain the growth of scientific knowledge, it can also explain how facts are overturned.

ANYONE
who was alive during the Late Cretaceous, between about one hundred million and sixty-five million years ago, would have seen many familiar creatures: the fearsome tyrannosaurs; duck-billed hadrosaurs; numerous birds; and even a selection of small mammals. If you ventured into the sea, you would have seen a wide variety of marine creatures. Among these were animals known as coelacanths (pronounced “SEE-luh-canths”)—gray, thick-lipped fish, they were one of the more hideous creatures of this period. A large meteor struck the Earth at the end of the Cretaceous period around what is today Mexico’s Yucatán peninsula, numerous volcanoes erupted that deposited ash across the sky, and the planet was plunged into a cataclysmic climate shift that caused the extinction of all of these creatures. In addition to wiping out our beloved dinosaurs, this massive extinction included the coelacanth, the ugly stepsister to nearly everything else that lived in the ocean.

But in 1938, on the eve of World War II, this changed. While the cause of the massive extinction at the boundary between the Cretaceous and Tertiary periods wasn’t yet known, the numerous extinct species from that time had already been chronicled. From the
Tyrannosaurus rex
to the coelacanth, what was lost in the past had been well studied. At the time, Marjorie Courtenay-Latimer, a young woman in South Africa, was curator of a small museum in
the town of East London, not far from Cape Town. She had befriended a fisherman in the area, and he would periodically show his catch to her, allowing her to add any possible finds to the museum’s collection.

One winter day at the end of 1938, when Courtenay-Latimer went to the dock to check out the fisherman’s haul, she noticed a strange fin poking out. When she excavated it from the rest of the pile she discovered what she described as “the most beautiful fish I had ever seen, five feet long, and a pale mauve blue with iridescent silver markings.” No doubt this was the scientist in her speaking, with the thrill of seeing something possibly new. Anyone else would have seen a terribly ugly, oily, and foul-smelling fish.

That was definitely the assessment of a taxi driver when Courtenay-Latimer attempted to catch a ride back to the museum with her large, stinking find. But her hunch that this fish was important was verified when she scoured her books back at the museum and identified her catch as the long-lost coelacanth. Somehow, Courtenay-Latimer was astonished to discover, this fish had survived in the Indian Ocean unchanged by evolutionary pressures for tens of millions of years. This was confirmed by a professor at a nearby university, who upon seeing sketches of her find sent the telegram
MOST IMPORTANT PRESERVE SKELETON AND GILLS = FISH DESCRIBED
.

It took another decade and a half, and the offer of a large reward, for a second coelacanth specimen to be discovered. This one was found off the Comoros islands, between Madagascar and the African mainland. But the impossible had been done: A supposedly extinct species had been discovered alive and well.

The coelacanth is an example of what are known as Lazarus taxa: living things that are presumed long extinct until contrary evidence is discovered. Of course, predicting whether a single extinct species will one day be rediscovered living in some corner of the planet is nearly impossible. But if we look at large groups of species we sometimes can determine, in aggregate, how many species might actually not be extinct after all, and how often facts are incorrect and need to be overturned.

In 2010, two biologists at the University of Queensland in Australia tabulated all the mammals that have very likely gone extinct in the past five hundred years. This yielded a list of 187 species. Then they checked to see how many were eventually recategorized as nonextinct. The answer: More than a third of all mammals that allegedly were lost to time in the past five hundred years have since been rediscovered.

This sort of large-scale analysis is not just for understanding the nature of Lazarus taxa. It can be extended more generally to enable us to understand the entire edifice of science and how we overturn long-held scientific beliefs. By looking at how science changes overall, we can see the patterns in how scientific knowledge is revised. And it can lead us to measuring the half-life of facts.

.   .   .

AS
scientific knowledge grows rapidly, it leads to a certain overturning of old truths, a churning of knowledge. While this churning is hard to deny—recall my inability to recall the health benefits of red wine despite having seen it in the newspapers many times—it is difficult to measure. But if we could quantify this churn, that could provide a handle for our uncertainty, and even a metric for how often we should revisit a subject.

A few years ago a team of scientists at a hospital in Paris decided to actually measure this. They decided to look at fields that they specialized in: cirrhosis and hepatitis, two areas that focus on liver diseases. They took nearly five hundred articles in these fields from more than fifty years and gave them to a battery of experts to examine.

Each expert was charged with saying whether the paper was factual, out-of-date, or disproved, according to more recent findings. Through doing this they were able to create a simple chart that showed the amount of factual content that had persisted over the previous decades. They found something striking: a clear decay in the number of papers that were still valid.

Furthermore, they got a clear measurement for the half-life
of facts in these fields by looking at where the curve crosses 50 percent on this chart: forty-five years. Essentially, information is like radioactive material: Medical knowledge about cirrhosis or hepatitis takes about forty-five years for half of it to be disproven or become out-of-date. This is about twice the half-life of the actual radioisotope samarium-151.

Figure 2. Decay in the truth of knowledge in the areas of hepatitis and cirrhosis. The 50 percent mark is around forty-five years, meaning it takes about forty-five years for half of the knowledge in these fields to be overturned. From Poynard, et al. “Truth Survival in Clinical Research: An Evidence-Based Requiem?”
Annals of Internal Medicine
136, no. 12 (2002): 888–95.

As mentioned earlier, while each individual radioactive atom’s decay is subject to a great deal of uncertainty, in the aggregate, they are far from random. They are subject to a systematic degradation and encapsulated in the shorthand of a single number—the half-life—that denotes how long it takes for half of the material to be subject to radioactive decay.

Knowledge in a field can also decay exponentially, shrinking by a constant fraction. It is like one of Zeno’s Paradoxes, according to which we keep getting halfway closer to the finish line but never
quite reach it. In this case, the finish line is the point at which no papers from the original batch of cirrhosis and hepatitis studies are still true. While there will always be an infinitesimal number of papers cited many decades, or even centuries, from now, within a certain number of years the vast number of articles will have decayed into irrelevance. Of course, some of these are not wrong, just obsolete. These scientists noted that the effectiveness of treatments in decades past doesn’t necessarily become nullified; they simply become superseded by something newer, such as novel vaccines that make treatment of a disease no longer necessary.

But ultimately, while we can’t predict which individual papers will be overturned, just like we can’t tell when individual radio active atoms will decay, we can observe the aggregate and see that there are rules for how a field changes over time. The “half-life,” whether mathematically rigorous or simply conceptual, captures the regularities behind how knowledge changes. In addition, these results are nearly identical to a similar 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 forty-five years. As the French scientists noted, all of these results verify the first half of a well-known medical aphorism by John Hughlings Jackson, a British neurologist in the nineteenth and early twentieth centuries: “It takes 50 years to get a wrong idea out of medicine, and 100 years a right one into medicine.”

This means that despite the ever-expanding growth of scientific knowledge, the publication of new articles, refutations of existing theories, the bifurcations of new fields into multiple subfields, and the messy processes of grant-writing and -funding in academia, there are measurable ways in which facts are overturned and our knowledge is ever renewed. I’m not simply extrapolating from this half-life of medicine to argue that all of science is like this. Other studies have been performed about the half-lives of different types of scientific knowledge as well.

Unfortunately, convening a panel of experts and having them comb through all of science’s past conclusions and giving a thumbs-up or thumbs-down to a paper’s validity isn’t quite feasible.
So we have to sacrifice precision for our ability to look at lots and lots of science relatively quickly. One simpler way to do this is by looking at the lifetime of citations. As mentioned before, citations are the coin of the scientific realm and the metric by which we measure the impact of a paper.

Most papers are never cited. And many more are cited only once and then forgotten. Others are only cited by their own authors, in their own other papers. But—and this is no doubt a point in favor of the scientific endeavor—there are numerous papers that are cited by others in the field. And there are the even rarer papers cited so many more times than those around them that they are truly fundamental to a field, towering well above other publications.

To understand the decay in the “truth” of a paper, we can measure how long it takes for the citation of an average paper in a field to end. 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. It is out-of-date. 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.

This gives us a sense of how knowledge becomes obsolete, but it also has a very practical application. Scholars in the field of information science in the 1970s were concerned with understanding the half-life of knowledge for a specific reason: protecting libraries from being overwhelmed.

In our modern digital information age, this sounds strange. But in the 1970s librarians everywhere were coping with the very real implications of the exponential growth of knowledge: Their libraries were being inundated. They needed ways to figure out which volumes they could safely discard. If they knew the half-life of a book or article’s time to obsolescence, it would go a long way to providing a means to avoid overloading a library’s capacity. Knowing the half-lives of a library’s volumes would give a librarian a handle on how long books should be kept before they are just taking up space on the shelves, without being useful.

So a burst of research was conducted into this area. Information scientists examined citation data, and even usage data in
libraries, in order to answer such questions as, If a book isn’t taken out for decades, is it that important anymore? And should we keep it on our shelves?

Through this we can begin to see how the half-lives of fields differ. For example, a study of all the papers in the Physical Review journals, a cluster of periodicals that are of great importance to the physics community, found that the half-life in physics is about 10 years. Other researchers have even broken this down by subfield, finding a half-life of 5.1 years in nuclear physics, 6 years for basic solid state physics, 5.4 years in plasma physics, and so forth. In medicine, a urology journal has a half-life of 7.1 years, while plastic and reconstructive surgery is a bit more long-lived, with a half life of 9.3 years (note that this is far shorter than the half-life of 45 years calculated earlier, because we are now looking only at citations, not whether something has actually been disproved or rendered obsolete). Price himself examined journals from different fields and found that the literature turnover is far faster in computer science than psychiatry, which are both much faster than fields in the humanities, such as Civil War history.

Different types of publications can also have varied half-lives. In 2008, Rong Tang looked at scholarly books in different fields and found the following half-lives.