Dane Cook. Photo: Jared Milgrim/Corbis

Over the past 20 years, attempts have been made to understand through experimentation a phenomenon known as “cryptomnesia,” whereby you arrive at an apparently original idea that you later turn out to have heard from someone else or to have read somewhere. It can occasionally be the cause of what has perhaps rather generously been called “unconscious plagiarism.” In the laboratory, cryptomnesia can be replicated through subtle manipulation of the processes of forgetting. The trick is to mix just enough forgetting with remembering at just the right moment, such that the memory concerned does not disappear but is no longer recognized as memory.

Cryptomnesia, which means “hidden memory,” is fairly common. A person may be firmly convinced he or she has thought of a solution that was in fact proposed by a colleague at an earlier meeting. Cryptomnesia has been suggested as an explanation for the publication in a plastic surgery journal of a “new” surgical technique that in reality had been part of the training given to surgeons over many years. Rather more everyday examples might include thinking you had invented a new cocktail, an original pun or an exercise for basketball training. Bitter conflicts can arise in the workplace as a result of our tendency to come up with other people’s ideas from time to time.

The most famous example of cryptomnesia concerns George Harrison. In 1969 he wrote “My Sweet Lord,” which topped the charts worldwide. A writ was issued against him by the production company Bright Tunes, pointing out the song’s resemblance to “He’s So Fine,” a major number one hit for the Chiffons in 1963. Bright Tunes accused Harrison of plagiarism, saying the melodies of the two songs were virtually identical. Harrison admitted that he knew the number by the Chiffons but denied having copied it. The judge – perhaps he had dabbled in psychoanalysis – issued a verdict that was worded in such a way as to spare Harrison’s feelings. It must, he said, be a case of “inadvertent copying of what was in Mr Harrison’s subconscious memory.”5 But unintentional or not, it was still copying. Harrison was ordered to pay more than half a million dollars in royalties. To be done with the whole business he later bought the rights to “He’s So Fine.” Anyone clicking on that song on YouTube can indeed sing along to it with the lyrics of “My Sweet Lord.”

In 1989, two American psychologists, Alan Brown and Dana Murphy, attempted to gain an experimental grip on cryptomnesia. They developed an approach that set the pattern for later researchers. In the first session, test participants are asked to come up with ideas. Typically they have to brainstorm about solutions to complex problems, such as a medical diagnosis. During the second session, several weeks or months later, they are required to indicate what their contribution was on the previous occasion. In a third and final session they are challenged to come up with ideas that have not been proposed before.

Several dozen experiments along these lines have been carried out over the past 20 years and every time it turns out that in the second session participants tend to present other people’s ideas as their own. In the third session they often come up with ideas that they think are new but in reality have been proposed before, usually by one of the other participants. The cause is not simply confusion about ownership, since occasionally participants attribute their own ideas to others, although the number of times that happens is negligible compared to the times when ideas generated by other people are claimed by participants as their own. All this happens in good faith. Even when test participants have the chance to win considerable sums of money by attributing ideas correctly (and know this in advance), they still claim now and then to be the source of other people’s ideas. Sincerity is not the same as impartiality. The unavoidable consequence is that all individuals gain the impression that cryptomnesia is mainly something their colleagues suffer from. People who repeatedly see themselves cheated out of their own ideas must surely feel like an honest person surrounded by thieves.

The basic form of this kind of experiment is borrowed from earlier research into “source amnesia,” or forgetting the origin of things you recall. Source amnesia can mean that you tell a piece of juicy gossip you have just heard to a number of different acquaintances and eventually to the person who told it to you (who, you suddenly recall, impressed upon you that you must tell no one else). Cryptomnesia is rather different. With source amnesia you forget the origin of what you have heard or read, but you remember that there was a source. In a pure and authentic case of cryptomnesia you forget even that. The brilliant plan, the great invention, the timely bright idea that suddenly comes to you actually has its origins in your memory yet is not recognized as a memory.

It is of course true that forgetting who or what the source was will take you a long way in the art of coming up with other people’s ideas. Research shows that the factors that contribute to source amnesia increase the likelihood of cryptomnesia as well. The longer the interval in time, the more cases of cryptomnesia there will be. If the sources closely resemble each other, because they are all fellow students for example, this too makes cryptomnesia more common. Even sex makes a difference; it is easier for women to appropriate the ideas of other women, men those of other men. There is a “next-in-line” effect: those who came immediately before you in the brainstorming session run a slightly higher risk of having their ideas stolen by you, probably because you were already thinking about your own contribution as you listened. Ideas presented in chaotic circumstances, such as during brainstorming sessions or disorganized meetings, are also more likely to be incorrectly claimed at a later date.

Most likely of all to increase the occurrence of cryptomnesia is an invitation to participants to improve on ideas already proposed. Nothing separates the true owner so rapidly and efficiently from his or her ideas as a minimal addition or insignificant variation. Even after just a few weeks, the slightest contribution is sufficient to convince you that you have improved upon an idea of your own.

The identification of factors that contribute to cryptomnesia is not the same, of course, as an explanation. What actually happens?

Imagine you were at a meeting last week about some kind of complicated problem to which you proposed a clever solution. Then the conversation moved on, no one picked up on your idea, a colleague presented a different solution and the decision was made to try that first. At the next meeting it becomes clear that the solution is not going to work. Fortunately your colleague has come up with a different idea in the meantime, a better idea, a brilliant idea: yours. You glance around the table. To your horror you see that you are the only person who realizes it was your idea. What has gone on in your colleague’s brain?

A case like this is the result of that intriguing difference between two types of memory: semantic memory and autobiographical memory. The semantic memory contains material we might tend to call “knowledge” rather than recollection, such as knowing what “incubation” means, what a “joint and reciprocal will” is, or what the stretch of water between Britain and France is called. You once acquired that knowledge, but in most cases you will have forgotten the circumstances in which you learned it. Few people can say how or when they discovered that Stockholm is the capital of Sweden.

Autobiographical memory deals with the things we experience. This type of memory records the circumstances, or at any rate makes a valiant attempt to do so. Over time all kinds of things may be forgotten, but often you can still recall where something happened or who was there, whether it took place in the evening or during the day, at home, outdoors or at work. Recollections drawn from our autobiographical memories usually have a context.

When you presented your idea at the previous meeting you remained for a very short time in your colleagues’ autobiographical memories as part of their recollection of the suggested solution. The solution itself went into their semantic memories and there, with yourself as the rapidly fading context, it became linked up with all the knowledge they already had of the issue at hand.

Cryptomnesia is therefore not simply the consequence of a failing memory. It arises because a different part of the memory – in the case of your colleague the semantic memory, in the case of George Harrison the musical memory – has retained it extremely effectively. It is the discrepancy between the two memory processes that produces cryptomnesia. Seen from a broader evolutionary perspective, there is something to be said for such an arrangement. Confronted with a problem, it is not particularly helpful for the survival of the individual or group to remember just who came up with the brilliant idea, but it may prove invaluable to be able to recall what it was.

Adapted from Forgetting: Myths, Perils and Compensations by Douwe Draaisma, published in 2015 by Yale University Press. Copyright ©2015 by Douwe Draaisma. Reprinted by permission of Yale University press.