One of the central figures in the study of dreams is Calvin Hall, an American psychologist who developed the cognitive theory of dreams in the 1950s. Hall’s theory was based on his study of several thousand dream reports from ordinary people living all over the world, which he analysed for common themes and ideas.

His conclusion was that all dreams are based around three common concepts: the idea of the self; the idea of friends, family and other people; and the idea of the social environment.

Since then, the progress in dream research, or oneirology, has been slow. Despite the universal experience of dreaming, psychologists and neuroscientists have little understanding of its purpose or its mechanisms. So new approaches in this field are welcome.

Today, Onur Varol and Filippo Menczer at Indiana University in Bloomington, use network science to study the similarities between dreams reported in English, Chinese and Arabic—research that is funded in part by the Defence Advanced Research Projects Agency or DARPA.

They say their results provide a new window into the cultural links between dreams experienced by people in different parts of the world. What’s more, the work clearly back Hall’s cognitive theory.

Network science has changed the way researchers study a wide range of disparate subjects. The networks formed by the web of links between friends and contacts have become well-known thanks to the explosive emergence of social networking sites.

Less well known is the fact that similar networks exist between characters in many important works of literature, such as the Odyssey in the Iliad, various fairy tales and the mediaeval Icelandic sagas. Network science is also providing unprecedented insight into the way biomolecules interact inside body and hence into the nature of disease; into connectivity between regions in the brain; and even into the links between ingredients in recipes from different parts of the world.

Dreams have recently become amenable to this kind of study because dream reports and their interpretations are now widely available on the web. For example, the DreamBank database compiled at the University of California, Santa Cruz, provides a description of around 1400 symbols that appear in some 20,000 dream reports in English. Examples of symbols include ‘love’, ‘ladder’, ‘quarrel’, ‘voices’ and so on. Similar databases also exist for Chinese and Arabic.

Varol and Menczer began by crawling these databases for symbols and their descriptions. They then measured the similarity between symbols by comparing their descriptions. Next, they created a network for each language in which symbols are nodes and drew links between symbols that were similar.

The result was three networks linking the dream symbols found in English, Chinese and Arabic.