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The brains of twins are structurally similar to one another and are under relatively tight genetic control. Given the similarity in brain volume and structure of twins' brains should we expect similarity in dream content of twins? I would say yes but there are very little data out there to decide the issue.
Heritabilities (i.e., the proportion of phenotypic variance due to genetic versus environmental sources) of connectivity structures and overall brain volumes of twin's brains run in the range of .60 to .80 (Schmitt et al., 2008). These coefficients are relatively large and rival the high heritability coefficients for IQ.
Regardless of the strength of the relationship between the brains of two genetically similar or identical individuals it seems reasonable to suppose that similar brains should give rise to similar dream content if brains give rise to mental content in general.
Twins present the best possibility for finding two individual who exhibit very similar brain volumes and brain structure. So do twins exhibit similar dream content? We do not know. Many twins say that their dreams are in fact similar in content. Moreover many twins make a far stronger claim: They claim that they actually share the dreaming process between them!
In her recent dissertation "Do Twins Dream Twin Dreams? A Quantitative Comparison With Singles' Dreams," Margery H. Runyan, reported the following dream sharing accounts from twins: "One twin set reported nightmares with a similar threat, tornadoes, in recent dreams. One twin set reported the same occurrence from two points of view while sleeping side by side; one twin dreamt that she came to tell the other twin that she was not getting married, and the other twin dreamt that her twin came to tell her that she was not getting married." (p. 141).
In addition to these shared dreams Runyan reported apparent precognitive dreams where one twin foresaw in his dreams a calamity that was going to befall the other twin:
"For a few weeks, I had a recurring dream (nightmare) where I was driving my car after dark. During this period of driving, I would be involved in an automobile accident where the car I was driving would flip over and over again causing my death. I was almost to the point of not driving any more after dark when my identical twin died in an auto accident where he flipped his car over with it landing on the top directly over him. This caused him to have a massive brain injury resulting in his death." (Runyan, p. 140).
Perhaps even more importantly from an experimental point of view, Runyan quantitatively compared the dream content of twins with the dream content of singletons. Runyan used the standardized Hall - Van de Castle rating scales for dream content so that she could compare frequencies of various content items across the dreams of twins and the dreams of singletons.
Not surprisingly twin dreams are not entirely like the dreams of singletons. Some 44% of twin dreams contain the other twin as a character in their dreams. Singletons do not dream that frequently of their siblings. Indeed only 19% of characters in singleton dreams are family members (parents, siblings etc). Given that dreams of twins are often about their twin it is not surprising that social interactions in twin dreams are more friendly than what you see in singleton dreams. Friendliness appeared in 66% of the twin dreams but in only 42% of the singleton dreams.
Most interesting perhaps was the finding that 76% of the twin dreams took place in unfamiliar settings while only 38% of the singleton dreams did so. Why is this the case? Why should twins dream of unfamiliar surroundings far more than do singletons?
What do settings or surroundings signify in dreams? We do not know. Presumably characters in dreams represent the same characters as their waking counterparts. When a twin dreams about her twin that character does indeed represent the other twin-that should be the default hypothesis because it is the simplest hypothesis. But what about settings? A singleton tends to dream about familiar settings but twins do not. Given that twins dream a great deal about familiar CHARACTERS perhaps they use background settings to play around a bit.
Thus when listening to a twins' dreams we should notice (if this account is correct) that all the creative stuff would be in the settings rather than in the characters themselves. But that is only a guess. We really do not know.
Twin dreams represent a completely unexplored realm of the human Mind and of dreams. they represent a very rich area for science as they will reveal clues as to the nature of dreaming, as well as the nature of the brain -- mind relation itself. twin dreams reveal something of the capacity of dreams to extend the human mind beyond its typical functional limits.
Who else but twins can boast of shared dreams, precognitive dreams and dreams filled with uncannily unfamiliar settings?
References
Do Twins Dream Twin Dreams? A Quantitative Comparison With Singles' Dreams, (2010; UMI Number: 3389215 ProQuest LLC 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, MI 48106-1346)
