What counts as normal, whether a body temperature, a healthy reef or a heat wave at sea, is a choice scientists make rather than a fact they find, Elis Jones, a philosopher of science, argues in a new paper in the journal Biology & Philosophy.
Medicine settled on a normal body temperature of 37 degrees Celsius, or 98.6 degrees Fahrenheit, in the 1800s. But American men born in the early 1800s ran about 0.6 degrees Celsius, roughly a degree Fahrenheit, warmer than men do today, one major study found, and among the Tsimane people of the Bolivian Amazon the average fell from about 37 degrees in the early 2000s to about 36.5 by 2018. Either 37 is no longer normal, or the cooler readings are.
Every such question rests on a baseline, a picture of how things normally are that gives each ‘after’ a ‘before.’ Someone has to choose that picture, writes Dr. Jones, of the Konrad Lorenz Institute in Austria and the Technical University of Munich, whether the work involves regenerating animals, brain scans, unemployment figures or pandemic death counts. And the world keeps moving under the choice.
Daniel Pauly, a fisheries scientist, gave one version of the problem a name: shifting baseline syndrome. On many Caribbean reefs, overfishing removed the fish that graze on algae, leaving one species of sea urchin to do the job; when disease killed nearly all those urchins in the 1980s, algae spread. Each new generation of observers can take the damaged place it first meets as normal, and the losses fall from view.
The coral scientist Jeremy Jackson argued that baselines should reach back to pristine seas, before people changed them. But pristine keeps receding: different researchers date it to the 1970s, to a few centuries ago or to 12,000 years back, and by some old yardsticks an algae-covered reef might look more pristine, not less. History ‘cannot, on its own, determine what counts as a normal reef,’ Dr. Jones writes. Values fill the gap: picking a past state as normal means ‘that particular historical state is taken to be valuable.’ None of this makes today’s damaged reefs fine, the paper adds, and the past remains a useful record of change.
The same choice shows up at laboratory scale. Hydras, tiny freshwater animals with six to eight tentacles, can regrow from a cut-up piece, but deciding that one has regrown requires a picture of a normal hydra. The researchers Erez Braun and Hillel Ori used an electric field to fold adult hydras back into the hollow ball of cells each had grown from; when the field eased, the balls developed into adults, some with a different number of tentacles. The team counted a hydra as regrown once tentacles appeared, whatever their number, and did not count the folded ball as regrowth. Neither definition needs to claim the ‘true’ normal, Dr. Jones writes; each fits a different job.
Each picture is also an idealization, tidier than any real animal, and Dr. Jones argues that reef baselines are no different. They lump many species into ‘coral’ and ‘algae,’ and they smooth a tangled history into one in which coral always ruled. Even the normal past is partly made, not found.
The oceans pose the problem in real time. Scientists commonly call a patch of sea a marine heat wave when it stays hotter than nine out of 10 past readings there for that season, a baseline that often reaches back 30 years. As the water warms past the old line and stays there, a heat wave that never ends stops being a wave. Keep the old baseline and the slow climb comes through but the sharp bursts blur; reset it, and the bursts return while the climb folds into normal. Corals die past fixed temperatures, so the climb matters most to them. Fishing crews care about the sudden hot spells.
The oceanographers Amaya and colleagues have proposed keeping the word ‘heat wave’ for the sudden spikes and giving the slow warming a name of its own. Their article puts the puzzle plainly: ‘The question is, what is ‘normal’?’ Dr. Jones’s answer reframes shifting baseline syndrome as a quarrel over resets: the usual version assumes one true baseline, while in this telling, calling someone guilty of the syndrome means holding up another baseline and saying the reset was done the wrong way.
Whether medicine should retire 37 degrees depends on why the readings fell. Protsiv and colleagues, who ran the American study, judged thermometer error unlikely, because even among Civil War veterans examined in the same period, men born earlier ran warmer than men born later. If bodies truly changed, that team considered less inflammation the likeliest cause, pointing to fewer chronic infections and wider use of anti-inflammatory drugs. In that case, Dr. Jones suggests, the old 37 may describe sicker bodies, and ‘yesterday’s healthy human might be today’s unhealthy one,’ unless a little inflammation turns out to be good for us.
The Tsimane complicate the story: infections did not explain their drop, a team led by Gurven found, and if the cause lies in gut microbes, which some researchers suspect help keep people warm, agreeing on a new normal may be harder. Depending on the comparison, Dr. Jones writes, ‘a person can have a normal and an abnormal body temperature at the same time.’
The question recurs across fields. Brain researchers must define normal activity before reading a scan. Economists hunt a ‘natural’ rate of unemployment. In the Covid pandemic, the count of excess deaths depended on the baseline chosen. Should a resting pulse be judged against others of the same species, age and sex, the paper asks, or adjusted for that person’s ‘current level of caffeination, physical activity, or existential dread’? Each answer yields a different normal, and a baseline built for one job is easily misread in another as a verdict on what is good in general.
As people change the world faster, Dr. Jones expects that ‘debates around re-baselining may become a more common problem in many areas of science.’