Re: Are identical twins both right or left minded, or opposite minded?

Hi Don,
 Thank you for your interesting question about personality differences 
in identical twins. If I understand correctly, you say that two "young girl 
twins" that you know from work seem to have "very different 
personalities and attributes" and you link these differences to the 
idea that some people are "right-minded" and others are 
"left-minded".  
 This idea is based in our knowledge that the two hemisphere of the 
brain are not identical (brain asymmetry) and in demonstrations that 
some mental processes occur more in one hemisphere than in the 
other (hemispheric lateralization of function). You know that  
the left hemisphere of the brain controls the right hand (and vice 
versa), and the fact that most humans are right-handed (90%) 
indicates that in most humans, the left brain is dominant. Another 
classic example of this is that in most humans, language is 
produced and understood by the left hemisphere (for a good review 
of this, see Hellige, 2001). 
 As research accumulated on this subject of the lateralization of 
functions, a rather stereotyped view of the hemispheres progressively 
emerged. According to this, the left hemisphere is cognitively 
stronger and more logical, while the right hemisphere is more 
emotional and creative. 
 Personality theorists used these distinctions to propose two distinct 
personality types, the so-called left-minded and right-minded, which 
were then blown out of proportion by popular psychology as an easy 
way to categorize and understand people. Am I right in thinking that 
one of your twins is organized and efficient while the other is more 
artistic and maybe even a little eccentric?
 Indeed, solid evidence linking hemispheric dominance and 
personality type is very scarce. Although some links have been found, 
for example between left-handedness and schizophrenia and other 
psychopathic disorders (Springer and Deutsch, 1989), the concept of 
right- and left-minded as popular psychology presents it is not based 
upon neurological processes and hemispheric specialization.
 This leaves us with the other part of the question, which is whether 
twins tend to have similar or opposite personalities. It is generally 
agreed that almost all of the major personality dimensions have a 
substantial (50% more or less 20%) heritability (Revelle, 1995). In 
that sense, identical twins which share all their genes should be 
more similar in personality than fraternal twins or normal siblings. At 
the same time, these twins sharing 100% of their genes are never 
the exact same person. It nature vs nurture all over again!
 I hope this helps. Cheers to you and your twins!
   Benoit  
 
References

Hellige (2001) Hemispheric Asymmetry. Cambridge: Harvard 
     University Press.
Revelle (1995) Annual Review of Psychology 46, 295-328.
Springer and Deutsch (1989) Left Brain, Right Brain. New York: 
     Freeman and Co.