In China, people remember the period from roughly 1849 to 1949 as the “century of humiliation.” The time was turbulent, from the First Opium War (a defeat by the British) through many other defeats and unfavorable treaties in which Chinese people were dominated by the Japanese, French and English. Although the century was declared over in 1949 when the People’s Republic of China was established, the Chinese remember the sting of those times and still interpret modern events through them. For example, in 1999 during the NATO bombing of Belgrade as a part of the war in (former) Yugoslavia, U.S. smart bombs hit the embassy of the People’s Republic of China, killing three reporters. Chinese leaders were infuriated, calling it a “barbaric act” and a “violation of the Geneva convention.” Chinese people held huge rallies and demonstrations against the U.S. The U.S. claimed the bombing was an accident, guided by the C.I.A.’s faulty intelligence, and President Clinton apologized. For the Chinese, the bombing was a sharp reminder of the century of humiliation and fit the narrative of domination by the west, carried forward. A friend who was recently visiting China told his hosts that their remembrance of the embassy bombing was wrong, that the bombing was an accident. They looked at him with pity, saying “You can’t possibly believe that.” They saw him as another American duped by government propaganda.

Collective memory refers to how groups remember their past. The Chinese remember the century of humiliation, while Americans remember 9/11 and subsequent events, and the people of many nations remember the era of World War II. Collective memories may occur at more local levels, too. Families may remember their history or a particular salient event (e.g., a vacation in an exotic locale). Each of us has some sort of collective memory for any important social group to which we belong. These collective memories can be about facts or about interpretations, as in the remembrance of the embassy bombing.

To understand a country’s memories is to grasp something essential about their national identity and outlook. Of course, countries do not have memories; it is the people in the country who retain the memories, but often there are common themes. When asked to remember World War II, Americans report numerous events, but the majority of people report the attack on Pearl Harbor, D-Day and the bombings of Hiroshima and Nagasaki. When Russians are asked to list the critical events of World War II, they mostly list a different set of events, such the Battle of Stalingrad, where nearly 2 million soldiers were killed or captured as the Soviets repulsed the Germans, and the Battle of Kursk, the largest tank battle in history and another decisive Soviet victory. Russians do not even call it World War II; for them it is the Great Patriotic War.

The collective memories of a people can change over generations. A recent study showed that both younger and older Americans listed the U.S. bombings of Japan as a critical event in World War II. However, older adults (ones alive during the war) rated the bombings quite positively (the bombs ended the war; they spared American lives) whereas younger adults rated the bombings as negative (the bombs killed and injured thousands of civilians; the war would surely have ended soon anyway). When President Obama recently visited Hiroshima, U.S. news reports referred to similar shifting opinions about the bombings over the years, as assessed by public opinion polls since the war. Collective national memories are not fixed but change with the times.

Collective remembering implies that collective forgetting also occurs, and we have studied such forgetting in a particular context: how rapidly presidents are forgotten. Virtually every American can name the current president, and when doctors perform a quick neurological assessment of possible stroke or concussion, they ask the name of the current president to determine if cognition is somewhat intact. Because we can assume that the current president is known by virtually 100% of the population, we can then measure the forgetting that occurs when a president leaves office.

We measured recall of the presidents by college students in 1974, 1991 and 2009. The test simply asked them to recall as many presidents as possible in five minutes (with no cues) and to place them in order, if possible. Because of the length of time between our measurements, recall of the most recent presidents in 1974 (when they were nearly perfect) changed over time. Thus we could examine the forgetting rates of six presidents: Eisenhower, Kennedy, Johnson, Nixon, Ford and Carter. We found that Kennedy’s forgetting rate was slowest – he was best remembered over time – and that Nixon was also forgotten more slowly than the others. Eisenhower and Johnson, despite their great roles in American history, were forgotten rather rapidly. For example, in 1974, 97% of college students could recall Johnson as president, but by 1991 his recall had dropped to 71% and by 2009 he was recalled only 42% of the time.

On the other hand, perhaps Americans know the presidents, but they just cannot recall them on a 5-minute recall test. To examine this possibility, we gave a recognition test of the presidents to a large internet sample. We provided people 123 different names one at a time, which included 41 presidents (first and last names, like Zachary Taylor), vice presidents who did not become president (Levi Morton), other famous Americans (Benjamin Franklin) and finally some other names that sounded plausible (Thomas Moore) to round out the set. The people judged each name as either president or not a president and then rated their confidence on a sliding scale. Although recognition was far higher than the level of recall in prior studies, some presidents were still poorly recognized: Chester Arthur (who 46% of people identified), Franklin Pierce (56%), as well as Millard Fillmore and Benjamin Harrison (both 65%). And even when these presidents were correctly recognized, the subjects expressed low confidence.

The errors people made – that is, the false recognition of names of people who were not president – were of even more interest. To our surprise, 71% of our sample incorrectly recognized Alexander Hamilton as president and did so with relatively high confidence. Our test occurred before the musical Hamilton on Broadway and also before the announcement (later rescinded) that his image was to be removed from the ten-dollar bill. Benjamin Franklin and Hubert Humphrey were falsely recognized as president 39% of the time, with John Calhoun coming in at 37%. These cases of false recognition may be caused when raters are familiar with a name without knowing quite what the individual did – “I remember the name well from my history book, so he must have been president.”

Collective memory is a burgeoning topic of research, one that might be used to understand the perspective of people in other groups, whether of a nation or of a political party or other social group. In certain cases, we can also measure collective forgetting, as in our study of presidents. Those results reveal the evanescence of fame and provide a means to measure its decline. The memory of the most famous individual in a country for years will gently slide into oblivion over time. Extrapolating from our data, we estimated that Lyndon Johnson would be about as well remembered as James Polk in about 2054.