Can two or more people share the same dream? As far as I know, there have been no scientific investigations of this question. But there are literally thousands of well-documented accounts.
The best-documented cases involve therapist-client shared dreams. In these, there is a professionally trained therapist who verifies the claim that the dream happened to both the therapist and the client around the same time. The next-best documented cases involve people in close relationships like parent/children, spouses, or lovers. Consistent with the effect of emotional closeness on shared dreams, we also have plenty of well-documented cases of twins sharing the same dream. The least well-documented cases involve complete strangers sharing the same dream. (We only have anecdotal reports of strangers experiencing the same dream because the strangers happened to run into one another and recognize each another from the dream!)
I have written about the shared dreams of twins in this blog and cited sources on twins in that post. For sources on shared dreams between therapists and clients, see Anthony Shafton’s 1995 book Dream Reader. For sources on complete strangers sharing dreams, see Frank Seafield’s Dream Curiosities. You can also find forums on shared dreams all over the web. We have to conclude that people everywhere do occasionally experience the same dream as another individual.
What are we to make of this fact? First, all we have are anecdotal reports. People believe they experienced the same dream, but we have to remain skeptical until controlled scientific investigations are conducted. In addition, the two people involved never agree about every detail about the shared dream. Nevertheless, I have learned to respect anecdotal reports in the world of dream research because these reports are usually reliable. There is no incentive for people to lie about the experience.
There are some commonalities among the reports that increase confidence in their reliability. For example, most often the two people involved know each other and are emotionally close. Obviously, you are more likely to hear about unusual experiences involving two people if they see each other on a regular basis. In addition, the way in which the two people typically discover that they shared a dream is that one person begins sharing the dream without knowing that the other person had the same one until the other person jumps in and finishes it.
People often report that nothing unusual happened before the shared dream. They report that they did not talk about dreams with the other person before the event, so there is no indication of biasing or priming effects. The fact that the two people involved often do not agree about every detail in the dream actually increases my confidence that the reports are honest accounts. It seems inevitable that individual differences, ranging from mood to IQ to memory differences, prevent people from recalling every detail of a dream—so reports of a shared dream should vary accordingly. The small amount of variance concerning details in reports of shared dreams therefore makes sense. Interestingly, the timing of the event can vary as well. Sometimes the shared dream occurs at the same time for both people. In other cases, it does not. What is remarkable is that so much of the shared dream, sometimes including small details, are recalled as strikingly similar or even identical by the two people involved, regardless of the timing of their experiences.
So let us provisionally accept the fact of shared dreams: Two persons can have the same dream. What does this imply for the science of dreams? If we assume that brains produce dreams, we have to assume that the two brains involved were in the appropriate brain states to produce identical content in two people. This may mean that the two people must have been in identical brain states, and that these identical states produced the same cognitive content.
Yet this option seems almost impossible to me given the huge plasticity and variability in brain physiologies across individuals. Even the brains of twins are vastly dissimilar. So attributing shared dreams to coincidentally identical brain states seems a stretch. But other alternative explanations are equally unappealing: For example, two people having the same dream seems to suggest that dreams are not mere products of the sleeping brain. Instead, they arise outside of us and then “happen” to us. They are in some sense independent of the minds that record and express them. Dreams are perhaps products of the interpersonal cultural world and float in the cultural morphospace waiting to alight on an individual consciousness.
But if that were the case, why is it that the cultural memes manifest as shared dreams and not some identical cognitive content in waking life? Perhaps shared dreams are like abstract Platonic forms that are bigger than individual brains, so they are able manifest in several brains attuned to the form.
None of these possibilities seem appealing or plausible to me.
In short, we have no good explanations for shared dreams. Perhaps that is why science has not yet investigated these events. Science has no place to put them within its current worldview—but this is all the more reason to investigate them. Paradigm-challenging phenomena are the most important data for science because they force revolutionary changes.