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Environment and intelligence research investigates the impact of environment on intelligence. This is one of the most important factors in understanding human group differences in IQ test scores and other measures of cognitive ability. It is estimated that genes contribute about 20-40% of the variance in intelligence in childhood and about 60% in old age. Thus the environment and its interaction with genes account for the remaining approximate 40% of intelligence. Historically, there has been great interest in the field of intelligence research to determine environmental influences on the development of cognitive functioning, in particular, fluid intelligence, as defined by its stabilization at 16 years of age. Despite the fact that intelligence stabilizes in early adulthood it is thought that genetic factors come to play more of a role in our intelligence during middle and old age and that the importance of the environment dissipates. ==Neurobiological theory== As babies, our neuronal connections are completely undifferentiated. Neurons make connections with neighboring neurons, and these become more complex and more idiosyncratic as the child ages, up until the age of 16, when this process halts. This is also the time frame for development of what is defined in psychometric studies as the general factor of intelligence, or g, as measured by IQ tests. A person’s IQ is supposed to be relatively stable after they have reached maturity. It is likely that the growth in neuronal connections is largely due to an interaction with the environment, as there is not even enough genetic material to code for all the possible neural connections. Even if there was enough genetic material to code neural connections, it is unlikely that they could produce such fine tuned connections. In contrast the environment causes meaningful processing as the neurons adapt to stimuli presented.〔 The capacity of the brain to adapt its connections to environmental stimuli diminishes over time, and therefore it would follow that there is a critical period for intellectual development as well. While the critical period for the visual cortex ends in early childhood, other cortical areas and abilities have a critical period that lasts up through maturity (age 16), the same time frame for the development of fluid intelligence. In order for a person to develop certain intellectual abilities, they need to be provided with the appropriate environmental stimuli during childhood, before the critical period for adapting their neuronal connections ends.〔 The existence of a critical period of language development is well established. A case illustrating this critical period is that of E.M., a young man who was born profoundly deaf and did not have any interaction with the deaf community. At the age of 15 he was fitted with hearing aids and taught Spanish; however, after 4 years he still had severe difficulties in verbal comprehension and production. Some researchers believe that the critical period effect is a result of the manner by which intellectual abilities are acquired—that changes in neuronal connections inhibit or prevent possible future changes. However, the critical period is observed at approximately the same age in all people, no matter what level of intellectual ability is achieved.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「environment and intelligence」の詳細全文を読む スポンサード リンク
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