How much of a childs intelligence is genetically determined, inherited from his/her parents, and how much is due to his/her environment Discuss these important factors.
No set of genes or gene markers has been conclusively linked to the development of intelligence. Specific genes that have been studied are primarily those believed to be linked to the development of brain size.
Yet no link between human brain size and intelligence has been established. Other theories have been proposed to explain how difference in brain size and structure have evolved. One such theory detailed by British neuroscientist John R Skoyles in his paper, Human Evolution Expanded Brains To Increase Expertise Capacity Not IQ, argues that size differences are related to skill development Skoyles maintains that the human brain increased in size over time due to a need for increased expertise or skill capacity associated with highly complex tasks that have varied from culture to culture.
He argues that this explains difference in brain size among groups.
Contributing evidence comes from studies on children suffering from sever epilepsy who have had a brain hemisphere removed to prevent debilitating seizures. If removed early enough half a brain seems to work nearly as well as a whole one. The remaining hemisphere often co-opts the functions previously handled by the missing portion.
Data from patients suffering from microcephaly a congenital disorder which results in reduced brain size and function, reveals that though a majority of patients score considerably below average on IQ tests, a small portion test in the normal range.
These examples challenge the notion that a persons brain size determines his or her cognitive ability.
Some genes that have been correlated to G are those associated with cathepin D (CTSD) and cholinergic muscarinic 2 receptor (GHRM2). But it remains undear what role these particular genes play in the development of intelligence, if any. Genetic studies attempting to link genes to IQ have uncovered many candidates but little conclusive evidence. Research at this point has been restricted to analyzing levels of G and presence or absence of these candidate genes.
Findings from twin, adoption, and family studies are the most commonly cited forms of evidence for a biological theory of intelligence. These studies compare individuals with very similar DNA (identical twins or related familys members) with biologically unrelated children growing up In the same home or with children and their adoptive parents. This method attempts to distinguish traits a person is born with from those influenced by his or her environment. Molecular biologist, Robert Plomin has utilized such studies to estimate the heritability of of intelligence at around 50 (50%) of variance. Other studies utilizing G as a cognitive measure have arrived at similar estimates. Longitudinal studies show that these effects increase with age. The heritability of G appears to rise to about 75 (75%) by late adolescence.
One explanation for this shift is that family influence on cognition are deemed to diminish throughout development. Also possible, explains Plomin, is that additional gene expression delayed during childhood may be triggered as cognitive processes develop.
There are two significant problems associated with twin, adoption and family studies. First is the assumption that genetic effects can be separated from environmental effects. This position rests on the equal environments assumption (EEA) which posits that the environment of individuals in the same or different homes can be controlled for in such a way that genetic effects can be separated out. There have been serious critiques levied at EEA due to the way adoptive and non-adoptive environments are appraised as being different or alike.
Additionally the idea that genetic and environmental effects are simply additive and work in isolation of one another is false.
Second a majority of these studies do not account for how IQ outcomes are affected by class differences.
One study found that in families who subsisted on incomes at or below the poverty line, the heritability effects on IQ were close to zero, were as in affluent families these effects were quite high. They also found that parental education levels modified both the effects of heritability and environment, increasing the former and decreasing the letter as years of education increased. In cases where adequate nutrition access to education, protection from exposure to environmental toxins and similar issues have affected the development of individuals heritability estimates have been shown to be expressed quite differently.
A wealt of research has identified multiple environmental factors that may contribute to variation in IQ scores. Many studies have focused on variables such as nutrition, exposure to toxic chemicals, family environment, an how these might affect test scores. Quality and years of schooling have been shown to have an effect on IQ scores, for many children particularly those from low-income backgrounds school may be the primary transmitter of information.
One study followed a group of African-American students who were moved from a poor rural school in the south to an urban school with greater resources in Philadelphia. The groups average IQ score increased by one-half point each year that they attended the Philadelphia school. Ceci found a positive correlation between years of school attended and IQ scores. The study found that when the same age children enter school a year apart, those with the additional year of school have higher mean scores. These results are borne out in our society. Children who attend poor schools in rural or urban areas tend to score lower on IQ tests then those that have access to a higher quality of education. The effects of toxic substances such as lead and alcohol on IQ scores are well known.
Children living in impoverished environments are at much greater risk for exposure to both. Building codes and zoning lows have reduced lead levels in recent years but the effects of other toxic chemicals in these environments have not been well studied, fetal alcohol syndrome a condition resulting from the use of alcohol during pregnancy leads to a range of deficits in cognitive functions such as attention and memory as well as IQ. Studies conducted on children who have been abused and neglected show that this often results in a higher then average rate of psychiatric and cognitive disorders.
Even in homes with less extreme conditions, the differences in the use of language and size of vocabulary have been shown to effect scores on verbal IQ tests.
Parents years of schooling effect the IQ scores of children as does early exposure to concept such as counting. Most of the evidence on environmental contributors to intelligence scores and intellectual development show a clear advantage for children who grow up in middle class homes.