In the once fertile garden of epidemiology, all is not well, according to some commentators. The low-hanging fruit has been plucked, and the epidemiological ladder is not long enough to bring the remainder within reach. Possibly the most famous expression of this dissatisfaction is a report by a journalist writing in Science in 1995 called “Epidemiology Faces Its Limits”. Gary Taubes cites a number of contrary findings, where exposures have been found to be harmful and then safe (or vice versa) in different studies, or harmful in different ways, or harmful when studied using one study design but not when using another. He interviews a number of eminent epidemiologists and reaches a simple diagnosis: epidemiology has spotted the big effects already, and is now scrabbling around trying to identify small ones. These are much harder to distinguish from biases or chance effects. Indeed, he hypothesizes that epidemiological methods may be unable to tell the difference at all, in some cases. In this sense, Taubes suggests, epidemiology is facing its limits.
The epidemiological garden is still growing nearly two decades later. Either the gardeners did not listen, and continued to tend fruitless trees, or Taube’s diagnosis was wrong. But epidemiologists did listen: the piece is well-known. Moreover epidemiologists are among the most methodologically reflective and self-critical of scientists, which is evident from the fact that most of Taubes’ criticism is drawn directly from interviews with epidemiologists (and which is one reason epidemiologists are such a pleasure to engage with philosophically). The implication is that Taubes’ low-hanging fruit hypothesis is mistaken.
Taubes’ hypothesis is tempting because it is true that big discoveries lie in the past. It is, however, a fallacy to suppose that this means no big discoveries lie in the future. On inspection, the tempting hypothesis reveals itself as an instance of a very common theme: that we are nearing the end of what inquiry can tell us. This has been said before, most famously in physics shortly before Einstein’s impact. If the history of science tells us anything it is that this claim is always false. We know more about the past than the future, and so we know what the big discoveries of the past are, but not the big discoveries of the future. If there were low-hanging fruit that epidemiology has not yet plucked, then we would not know it, even if they were going to be plucked tomorrow afternoon. More is needed to prove that epidemiology faces its limits than that the tautologous claim that its most striking discoveries to date lie in the past.