Re: Is it true that the left brain is associated with masculinity ?

Although I am not an expert in the field of human neuropsychology, from 
what I've read so far no such correlation seems to exist.  Sounds like one 
of these popular distortions of scientic findings (a.k.a. modern myths) to 
me. 
   First of all, in normal human beings the brain hemispheres usually do 
not function in isolation from one another; usually they cooperate on most 
tasks. One hemisphere might be dominant but the other usually has at least 
a rudimentary capability, i.e. contributes a little bit. 
In addition, if anything at all, then it is your hormones and their (possible) 
influence on brain development, that make you masculine or feminine.  
This should affect the entire brain (including subcortical, i.e. more 
primitive areas, which contribute a lot to human behavior as well) and not 
a single hemisphere. 
   What you might be referring to is a hypothesis that's been around for 
some time. Testosterone, the male sex hormone, is believed to slow down 
development of the left hemisphere and thus account for the different brain 
organizations of males and females. This hasn't conclusively been 
proven or validated though. I suggest reading chapter 10 from the Kolb and 
Whishaw book (1996) and chapter 1 (pp. 43 to 56) from Tavris (1992) if 
you're interested in more details.

    Note that during the last 30 to 40 years differences (e.g. in the 
scores on collegiate admission tests) have vanished almost entirely. Men 
and women have virtually equivalent verbal and reasoning skills and the 
scores on mathematical ability tests differ only slightly. (Which might 
incite suspicion that earlier tests were either biased or that in our 
modern day society some differences level out due to environmental factors, 
e.g. co-ed education, exposure to technology in every-day life, etc.)
There is a noteworthy exception after all: men/boys by far outnumber women 
when it comes to exceptional mathematical giftedness. The reason for this 
has not yet been found. In addition, it does not give us a clue as to 
whether these persons will also succeed in life or in their jobs (or if 
they are likeable or not).
    One must consider that functional or physiological differences (brain 
organization, cerebral blood flow, brain activation patterns, etc.) do not 
necessarily show up as behavioral differences. The output may be equivalent 
or the same even though the inner workings vary! 
Scientific investigation into verbal and spatial abilities has produced 
incoherent/controversial results. Some researchers find more or less 
significant differences and others don’t. E.g. check this reference 
or this one. 
Here’s another selected one. Are these diverging outcomes a 
result of methodological differences (e.g. the kind of test used, sample 
bias, or the like)? Or do researchers interpret their results in a 
different way based on their personal assumptions/theories? 

After reading Carol Tavris's book 'The mismeasure of woman' I suspect that 
a great many of the gender differences one notices are probably not based 
on biology but rather on sociocultural factors such as education, 
(identification with) gender role , socialization, and opportunities, in 
general subsumed under the term of 'life history'. For example, one 
possible explanation for  differences in scientific knowledge between men and women 
 might be the degree of exposure to science and technology during 
childhood and early adulthood (teen-age and thereafter).

So the question remains: what makes us clever in general? Nature or nurture 
or an interplay of both?
To give you a tentative answer: Even if the sexes differ in their 
abilities they (and accordingly every individual as well) have to be 
treated in their own right. Trying to root up ‘significant differences’ 
based on biology can sometimes leave the impression of claiming a naturally 
justified superiority of one sex over the other. Sadly enough, such alleged 
scientific findings and popular beliefs have been abused to subdue/suppress 
women in the past.

In case you have any other questions, feel free to get back to me.

References:
1. Crucian GP, Berenbaum SA (1998): Sex differences in right 
   hemisphere tasks. Brain Cogn 36(3):377-89
2. Halpern DF, Haviland MG, Killian CD (1998): Handedness and sex 
   differences in intelligence: evidence from the medical college admission 
   test. Brain Cogn 38(1): 87-101
3. Jaeger JJ, Lockwood AH, Van Valin RD Jr, Kemmerer DL, Murphy BW, Wack 
   DS (1998): Sex differences in brain regions activated by grammatical 
   and reading tasks. Neuroreport 9(12):2803-7.
4. Kerns KA, Berenbaum SA (1991: Sex differences in spatial ability 
   in children. Behav Genet 21(4):383-96
5. Kolb B and Whishaw IQ (1996): Fundamentals of Human 
   Neuropsychology. WH Freeman and Company.
6. Kramer JH, Delis DC, Kaplan E, O'Donnell L, Prifitera A 
   (1997): Developmental sex differences in verbal learning. 
   Neuropsychology 11(4):577-84
7. Lynn R (1998): Sex differences in intelligence: a 
   rejoinder to Mackintosh. J Biosoc Sci 1998 Oct;30(4):529-32; discussion 
   533-9
8. Nagae S (1985): Handedness and sex differences in the processing 
   manner of verbal and spatial information. Am J Psychol 98(3):409-20
9. Stumpf H, Klieme E (1989): Sex-related differences in spatial 
   ability: more evidence for convergence. Percept Mot Skills 69(3 Pt 
   1):915-21
10. Tavris C (1992): The mismeasure of woman. Simon and Schuster
11. Wisniewski AB (1998): Sexually-dimorphic patterns of cortical 
   asymmetry, and the role for sex steroid hormones in determining cortical 
   patterns of lateralization. Psychoneuroendocrinology 23(5):519-47