Consensus is a necessary step on the road from scientific hypothesis to theory. What many people don’t realize, however, is that a consensus isn’t necessarily the last word. A consensus, whether newly proposed or well-established, can be wrong. In fact, the mistaken consensus has been a recurring feature of science for many hundreds of years.

A recent example of a widespread consensus that nevertheless erred was the belief that peptic ulcers were caused by stress or spicy foods – a dogma that persisted in the medical community for much of the 20th century. The scientific explanation at the time was that stress or poor eating habits resulted in excess secretion of gastric acid, which could erode the digestive lining and create an ulcer.

But two Australian doctors discovered evidence that peptic ulcer disease was caused by a bacterial infection of the stomach, not stress, and could be treated easily with antibiotics. Yet overturning such a longstanding consensus to the contrary would not be simple. As one of the doctors, Barry Marshall, put it:

“…beliefs on gastritis were more akin to a religion than having any basis in scientific fact.”

Earlier last century, an individual fighting established authority had overthrown conventional scientific wisdom in the field of geology. Acceptance of Alfred Wegener’s revolutionary theory of continental drift, proposed in 1912, was delayed for many decades – even longer than resistance continued to the infection explanation for ulcers – because the theory was seen as a threat to the geological establishment.

Geologists of the day refused to take seriously Wegener’s circumstantial evidence of matchups across the ocean in continental coastlines, animal and plant fossils, mountain chains and glacial deposits, clinging instead to the consensus of a contracting earth to explain these disparate phenomena. The old consensus of fixed continents endured among geologists even as new, direct evidence for continental drift surfaced, including mysterious magnetic stripes on the seafloor. But only after the emergence in the 1960s of plate tectonics, which describes the slow sliding of thick slabs of the earth’s crust, did continental drift theory become the new consensus.

A much older but well-known example of a mistaken consensus is the geocentric (earth-centered) model of the solar system that held sway for 1,500 years. This model was originally developed by ancient Greek philosophers Plato and Aristotle, and later simplified by the astronomer Ptolemy in the 2nd century. Medieval Italian mathematician Galileo Galilei fought to overturn the geocentric consensus, advocating instead the rival heliocentric (sun-centered) model of Copernicus – the model which we accept today, and for which Galileo gathered evidence in the form of unprecedented telescopic observations of the sun, planets and planetary moons.    

Although Galileo was correct, his endorsement of the heliocentric model brought him into conflict with university academics and the Catholic Church, both of which adhered to Ptolemy’s geocentric model. A resolute Galileo insisted that:

 “In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual.”

But to no avail: Galileo was called before the Inquisition, forbidden to defend Copernican ideas, and finally sentenced to house arrest for publishing a book that did just that and also ridiculed the Pope.

These are far from the only cases in the history of science of a consensus that was wrong. Others include the widely held 19th-century religious belief in creationism that impeded acceptance of Darwin’s theory of evolution, and the 20th-century paradigm linking saturated fat to heart disease.

Consensus is built only slowly, so belief in the consensus tends to become entrenched over time and is not easily abandoned by its devotees. This is certainly the case for the current consensus that climate change is largely a result of human activity – a consensus, as I’ve argued in a previous post, that is most likely mistaken.

Next: Nature vs Nurture: Does Epigenetics Challenge Evolution?