The press also played a critical role in shaping the evolution of the dietary-fat controversy by consistently siding with proponents of those who saw dietary fat as an unneccessary evil. These were the researchers who were offering specific, positive advice for the health-conscious reader—eat less fat, live longer. The more zealously stated, the better the copy. All the skeptics could say was that more research was necessary, which wasn’t particularly quotable. A positive feedback loop was created. The press’s favoring of articles that implied Keys’s hypothesis was right helped convince the public; their belief in turn would be used to argue that the time had come to advise cholesterol-lowering diets for everyone, thus further reinforcing the belief that this advice must be scientifically defensible.

Believing that your hypothesis must be correct before all the evidence is gathered encourages you to interpret the evidence selectively. This is human nature. It is also precisely what the scientific method tries to avoid. It does so by requiring that scientists not just test their hypotheses, but try to prove them false. “The method of science is the method of bold conjectures and ingenious and severe attempts to refute them,” said Karl Popper, the dean of the philosophy of science. Popper also noted that an infinite number of possible wrong conjectures exist for every one that happens to be right. This is why the practice of science requires an exquisite balance between a fierce ambition to discover the truth and a ruthless skepticism toward your own work. This, too, is the ideal albeit not the reality, of research in medicine and public health.

In 1957, Keys insisted that “each new research adds detail, reduces areas of uncertainty, and, so far, provides further reason to believe” his hypothesis. This is known technically as selection bias or confirmation bias; it would be applied often in the dietary-fat controversy. The fact, for instance, that Japanese men who lived in Japan had low blood-cholesterol levels and low levels of heart disease was taken as a confirmation of Keys’s hypothesis, as was the fact that Japanese men in California had higher cholesterol levels and higher rates of heart disease. That Japanese men in California who had very low cholesterol levels still had more heart disease than their counterparts living in Japan with similarly low cholesterol was considered largely irrelevant.

Keys, Stamler, and their supporters based their belief on the compelling nature of the hypothesis supplemented only by the evidence in support of it. Any research that did not support their hypothesis was said to be misinterpreted, irrelevant, or based on untrustworthy data. Studies of Navajo Indians, Irish immigrants to Boston, African nomads, Swiss Alpine farmers, and Benedictine and Trappist monks all suggested that dietary fat seemed unrelated to heart disease. These were explained away or rejected by Keys.