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Brain
The brains of many animals, including humans, are significantly different for males and females of the species. Both genes and hormones affect the formation of many animal brains before "birth" (or hatching), and also behaviour of adult individuals. Hormones significantly affect human brain development, as well as changes in the brain at puberty. It is also true that experience plays a critical role in the development of the brain, and studies of neuroplasticity show that experience can influence the structure and function of the brain throughout the lifespan.

In 2006, Alexandra M. Lopes and colleagues wrote that: A sexual dimorphism in levels of expression in brain tissue was observed by quantitative real-time PCR, with females presenting an up to 2-fold excess in the abundance of PCDH11X transcripts. We relate these findings to sexually dimorphic traits in the human brain. Interestingly, PCDH11X/Y gene pair is unique to Homo sapiens, since the X-linked gene was transposed to the Y chromosome after the human–chimpanzee lineages split.

It is commonly accepted that there are many sex-related differences in behavior in the human species. However, the neurobiological bases of these differences in behavior are unclear. There is confirmation by multiple studies (both MRI segmentation and 113xenon clearance method) that women have a higher percentage of grey matter (GM) in comparison to men, whereas men have a higher percentage of white matter (WM) and of cerebrospinal fluid (CSF). The percentage of GM in men was higher in the left hemisphere, the percentage of WM was symmetric, and the percentage of CSF was higher in the right hemisphere. Women, on the other hand, showed no asymmetries. Grey matter is composed of the cell bodies and dendrites of neurons. White matter is composed of neuronal axons and associated myelin. According to Gur and colleagues, it is thought that the higher proportion of GM in women could compensate for lower brain volume in women. The differences in percentage and asymmetry of GM and WM within the brain may account for the differences in cognitive functioning of the two sexes, although no one has yet established a clear causal link between the two. In 1999, Gur and colleagues claimed: The anatomic results suggest some parallels between sex differences in cognition and differences in GM because both women and the left language hemisphere have higher percentage of GM, and the women outperform men on language tasks..

Haier and colleagues used voxel-based morphometry (VBM) to identify brain areas where clusters GM and WM volumes are located in correlation to the FSIQ test. In 2004, Haier et al. claimed that: These findings support the view that individual differences in grey and white matter volumes, in a relatively small number of areas distributed throughout the brain, account for considerable variance in individual differences in general intelligence. However, both neuronal cell bodies (grey matter) and axons (white matter) are essential to the function of the nervous system, so the functional implication of having more of one or the other is not clear.

See Sex and Intelligence wiki for more information and research findings on this subject.