1. Ulcers Are Caused by Stress and Spicy Food
For most of the twentieth century, the explanation for peptic ulcers was almost universally agreed upon: too much acid, too much stress, too many rich meals. This was the commonly held belief – that peptic ulcers resulted from an excess of gastric acid released in the stomach as the result of emotional stress, the ingestion of spicy foods, or other factors. Doctors prescribed antacids, bland diets, and lifestyle changes. The idea that a bacterium might be the culprit was essentially off the table, because no one believed microorganisms could survive the stomach’s acid environment.
Barry Marshall and Robin Warren showed that the bacterium Helicobacter pylori plays a major role in causing many peptic ulcers, challenging decades of medical doctrine. To prove the point decisively when the scientific community resisted, Marshall did something extraordinary: he drank a solution teeming with H. pylori and developed gastritis within days. At the time Warren and Marshall announced their findings, it was a long-standing belief in medical teaching and practice that stress and lifestyle factors were the major causes of peptic ulcer disease. Warren and Marshall rebutted that dogma, and it was soon clear that H. pylori causes more than ninety percent of duodenal ulcers and up to eighty percent of gastric ulcers. The two men were awarded the Nobel Prize in Physiology or Medicine in 2005.
2. Dietary Fat Is the Primary Driver of Heart Disease
In the early 1950s, American cardiologist Ancel Keys proposed a simple idea: saturated fat raises blood cholesterol, and high cholesterol causes heart disease. The hypothesis spread quickly through government health agencies, and by the 1980s it had reshaped the food supply. By 1980, the low-fat approach became an ideology promoted by the media, physicians, and the government’s Dietary Guidelines, which encouraged consumers to reduce their intake of saturated fats.
The problem was with how the original data had been selected. Keys examined data from 22 countries and selected only seven to present in what became known as the Seven Countries Study. The countries he excluded were the ones that didn’t fit his hypothesis – countries where populations ate enormous amounts of saturated fat but had low rates of heart disease. Research published in JAMA Internal Medicine in 2016 revealed that in the 1960s, the sugar industry paid Harvard researchers to blame fat as the cause of heart disease, while discounting the role of sugar. The dominant theory had been, in part, a manufactured one.
3. Adult Brains Cannot Grow New Neurons
Ever since the dawn of modern neuroscience, there had been one seemingly absolute belief as laid out by Santiago Ramón Cajal, the father of neuroscience: in adults, brain cells are “fixed, ended, and immutable. Everything may die, nothing may be regenerated,” he proclaimed in the early 20th century. This view persisted through most of the twentieth century and was taught as settled fact in medical schools around the world.
For decades, medical textbooks declared that adults cannot grow new brain cells. Once your neurons died, that was it. A landmark study from Karolinska Institutet overturned this long-held belief, showing that adults – even into their late seventies – can generate new neurons in the hippocampus, the brain’s center for memory and learning. The debate is still active. One 2018 study found that after the first few years of life, neurogenesis in the human hippocampus is nonexistent or at least extremely rare. Yet research from Nature Medicine discovered that adults with healthy brains may produce thousands of new neurons daily, while production slowed significantly in those with Alzheimer’s. The original certainty, at minimum, is gone.
4. Most of Human DNA Is Useless “Junk”
About 95 percent of the human genome had been designated as “junk,” including most sequences within introns and most intergenic DNA. This label stuck for decades. The assumption was that evolution had left behind enormous stretches of genetic dead weight – leftovers from ancient viruses and mutations that served no useful purpose at all.
By far the best-known effort to identify functional regions in the human genome is the ENCODE project, whose authors made the remarkable claim that a “biochemical function” could be assigned to roughly eighty percent of the human genome. The findings triggered a significant scientific debate. New discoveries regarding potentially important sequences among the non-protein-coding majority of the genome are becoming more prevalent. Even those who dispute the precise figures acknowledge that some junk DNA may function in ways that are not currently understood. The word “junk” has largely disappeared from serious scientific discussion.
5. The Continents Have Always Been Where They Are
When Alfred Wegener proposed in the early twentieth century that the continents had once been joined together and had since drifted apart, he was largely dismissed. The scientific establishment found no plausible mechanism for how entire landmasses could move, and the idea was treated as fringe speculation for years after his death. Wegener died in 1930 on an expedition in Greenland. Poorly respected in his lifetime, Wegener and his ideas about moving continents seemed destined to be lost in history as fringe science.
In the 1950s, evidence started to trickle in that made continental drift a more viable idea. By the 1960s, scientists had amassed enough evidence to support the missing mechanism – namely, seafloor spreading – for Wegener’s hypothesis of continental drift to be accepted as the theory of plate tectonics. The vindication was total, and the reversal stands as one of the most striking examples in the history of science of a rejected idea eventually becoming the new orthodoxy. Ongoing GPS and earthquake data analyses continue to support this theory.
6. Dietary Cholesterol from Eggs Directly Causes Heart Disease
For much of the late twentieth century, the egg was something close to a dietary villain. Health authorities warned people to limit their consumption of whole eggs, particularly the yolk, because of the cholesterol content. For decades, it had been advised to restrict consumption of eggs, particularly yolk. This guidance filtered into everything from hospital meal plans to school lunch programs, and it shaped how an entire generation thought about breakfast.
The evidence, however, was always more complicated than the messaging suggested. Although eggs are affordable and nutrient-dense, there were concerns over potential cardiovascular complications associated with high dietary intake of eggs because of their cholesterol content. Further evidence, however, indicated that eggs were not significantly associated with adverse cardiovascular outcomes. What researchers came to understand was that dietary cholesterol behaves differently in the body than blood cholesterol, and that the two are not as tightly linked as once assumed. A 1965 literature review published in the New England Journal of Medicine, sponsored by the Sugar Research Foundation, singled out fat and cholesterol as the dietary causes of coronary heart disease and downplayed evidence that sugar consumption was also a risk factor – which had helped cement the cholesterol-fear narrative in the first place.
What these six stories share is not carelessness or bad faith on the part of scientists, but something more ordinary: the weight of consensus. Once an idea becomes the default assumption, it shapes which questions get asked and which data get noticed. The real lesson isn’t that science gets things wrong. It’s that the willingness to challenge what everyone already “knows” is what makes science work at all.