What makes and decides a human to be RIGHT HANDED or LEFT HANDED..?

its brain development.  Left handed people use the opposite hemisphere of their brains then right handed people. its a brain thing.  Left handed people are often more intelligent then right handed people on average.  and people that are ambidextrous use both hemispheres of their brains. Women use both hemispheres of their brains for problem solving wile men only one the left hemisphere. this is why when it comes to being ambidextrous you see more women then Men in that category.  its just a brain thing.  like some people have Long middle toes and others short. some people are just wired differently.

1) Right-handedness appears to be caused by an asymmetry in the human brain (in the planum temporale and the precentral gyrus).

2) These brain asymmetries appear to date back prior to the last common ancestor of chimps and humans, since not only do chimps and human share them, but also most chimps are also right handed.


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?In a second study, Hopkins and Cantalupo report the first-ever evidence of an association between hand preference and asymmetries in three areas of the brain cortex in chimps. Observing 66 chimps, they correlated asymmetries in brain anatomy with three measures of handedness: Simple reaching (which hand chimps used to pick up a raisin thrown into the cage), two-handed feeding (which hand chimps used to feed themselves chunks of fruit while holding the whole piece, such as a banana, in the other hand), and a measure of coordinated bimanual actions (which hand chimps used to fish peanut butter from a plastic tube with a finger).

Left-handed and right-handed chimps differed relative to the asymmetries in two primary motor areas, the planum temporale and the precentral gyrus. Say the authors, the results ?challenge the long-held belief that the neurobiological substrates for handedness are unique to humans.? Just as in humans, neuroanatomy governs whether a chimp becomes a lefty or a righty. Hopkins points out that chimps are also strongly right-handed for manual gestures and throwing, a clue to the origins of more general right-hand dominance in both chimps and humans.?

(?Just Like Us: Chimpanzee Brains Are Asymmetrical in Key Areas and Their Handedness Reflects It?, American Psychological Association, December 5, 2004
http://www.apa.org/news/press/releases/2004/12/chimp-brains.aspx
both retrieved 05/26/2010)
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?Researchers at Columbia, Mount Sinai School of Medicine and the National Institutes of Health have found that a region of the brain thought to control language is larger in one hemisphere in both chimpanzees and humans, disproving a theory that the brain section was asymmetrical only in humans.

The discovery, reported in the Jan. 9 issue of the journal Science, throws into question the role of the planum temporale, a part of the brain's temporal cortex that is located beneath the parietal cortex. The planum temporale of the left hemisphere is normally larger than in the right hemisphere in humans, but 94 percent of the chimpanzee brains studied demonstrated the same asymmetry.?
(Analogy in Chimp, Human Brains, Bob Nelson, Coumbia University Record, Vol. 23, No. 12, January 23, 1998  
http://www.columbia.edu/cu/record/archives/vol23/vol23_iss12/27.html
retrieved 05/26/2010)
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From birth you decide wheter you prefer your left or right hand. And which ever you choose is the one you use the most and becomes more dominant.