The following is my Genetics paper from semester
The origin of human intelligence has been the subject of scientific inquiry for many, many years, and with the research behind this exciting field picking up speed, pin-pointing the basis of human intelligence is becoming clearer and closer than ever. In this search for what makes us intelligent, there have been many discoveries, many obstacles and much that we have learned about our intelligence; but we still have yet to come across a solid conclusion to finally close the book once and for all on the inheritance, the genetic properties and the inner workings of intelligence. As far as we know there is no single gene for intelligence, be it IQ, reasoning, verbal skills, or memory. Intelligence has been found to be both genetic and environmental, a selective force in our evolution, and still a highly contested and researched area in the field of genetics.
To say that genes and environmental factors operate separately is totally false when it comes to the workings of intelligence, especially that of human beings. Many genes influence our intelligence, but not in the way that we would like to imagine. Most of the genes related to intelligence are round-about and they are genes whose primary function has little to do with IQ, memory, or reasoning. One of these genes uncovered in the early 1990’s was IGF2R, an insulin-like growth factor that is active only in the brain and is passed down maternally. If there was a link to the gene and intelligence, it would seem to suggest that intelligence was inherited from the mother; but upon duplication of gene manipulation, the results were disappointing. Other genes have been identified, such as INTLQ1, INTLQ2, and INTLQ3, which yield more promise for the future of intelligence genes. INTLQ1 is a quantitative trait locus, which is a region on a chromosome which contains one or more genes that contribute to a certain quantitative trait, such as intelligence. These genes all contribute minutely to the trait, but they work together to specify the trait.[1] INTLQ1 is found on chromosome 4 and contributes to cognitive abilities such as vocabulary, memory, and a small part of reasoning.[2] This QTL has been the most conclusive of the three, and the main researchers of this QTL, Devlin et al, have concluded that there is a heritability correlation of 48% of genetic inheritance evident in a correlational study. [3] Although, this study is not purely genetic as it has to include pre-natal conditions of those individuals in the womb and the behaviors of the mothers while pregnant. The other QTLs associated with intelligence, INTLQ2 and INTLQ3, are very loosely defined at this point, and very minimal influence has been shown to affect verbal ability by these genes. In order to further prove the influence of genes, scientists on the sorely “nature” side of the debate started twin studies, taking sample IQ tests of identical twins, fraternal twins, siblings and others in order to compare the tests of those with identical genes, similar genes and non-related genes.
Comparative IQ tests
% correlation
The Same Person Tested Twice 87
Identical Twins Reared Together 86
Identical Twins Reared Apart 76
Fraternal Twins Reared Together 55
Biological Siblings 47
Parents and Children Living together 40
Parents and Children Living Apart 31
Adopted Children Living Together 0
Unrelated People Living Apart 0
This correlational study seems to suggest that the genes behind intelligence are one of the driving factor in human aptitude and because there is no correlation with adopted children living together, environment seems to play a negligible role, but really, the conclusion is that intelligence is 50% inherited. Over time, the percent of inherited intelligence actually rises. As an individual grows up, they shed their influences and begin to express their own innate intelligence; a person’s genes drives then to pick their own environment based on their innate aptitudes.[4]
Comparative IQ tests
% correlation
The Same Person Tested Twice 87
Identical Twins Reared Together 86
Identical Twins Reared Apart 76
Fraternal Twins Reared Together 55
Biological Siblings 47
Parents and Children Living together 40
Parents and Children Living Apart 31
Adopted Children Living Together 0
Unrelated People Living Apart 0
This correlational study seems to suggest that the genes behind intelligence are one of the driving factor in human aptitude and because there is no correlation with adopted children living together, environment seems to play a negligible role, but really, the conclusion is that intelligence is 50% inherited. Over time, the percent of inherited intelligence actually rises. As an individual grows up, they shed their influences and begin to express their own innate intelligence; a person’s genes drives then to pick their own environment based on their innate aptitudes.[4]
Genes can also indirectly influence intelligence and how we learn. Certain genes which encode proteins that can help brain function and neural pathways will also affect intelligence and aptitude. Research indicates high intelligence related to how efficient the neural pathway is; genes that strengthen this pathway could contribute greatly to intelligence[5]. Genetic contributions to structure also can affect the way in which the brain works. Genes that strengthen mylinated sheaths could let the person who has them be able to transduce a message faster than others, recalling information faster and learning quicker. Genes that enhance cortical regions and make the regions larger in these individuals, could be able to be more adept at spatial reasoning than others, or have improved logical, mathematic or verbal skills because of increased brain volume. Also, a gene found to control the utilization of fatty acids called FADS2, can increase the IQ of children who are breast-fed as infants by an average of seven points[6]. This interaction between genes and environment confirms that intelligence is a multifactorial trait, and intelligence is not necessarily influenced directly by a single gene. Genes for other traits can also affect intelligence, such as diseases like PKU and Down’s syndrome can greatly impact the development of intelligence, either by metabolic consequences or chromosomal abnormality. Genes do not have to be connected to learning or aptitude in any way to be able to affect intelligence.
Intelligence can also be a product of evolutionary pressure, as seen in some groups of people throughout time. Ashkenazi Jews are a prime example of selective evolutionary pressure in intelligence. There has been major evidence to support the idea that Jews descended from the Ashkenazi line have a higher average IQ than other groups of people throughout the world.[7] The idea behind this theory is that because of strict religious law dictating marriage of other Ashkenazi Jews, the variation in the population goes down, restricting the different alleles available. Also, discrimination towards these people throughout history may have favored higher intelligence individuals, where only the cleverest of individuals survived. Historically, these people have also been assigned to mentally rigorous jobs such as banker and accountants and to do well in these jobs would have meant a higher life standard, which would be evolutionarily advantageous. But the Ashkenazim aren’t the only people in which intelligence is a selecting factor; in fact, all humans to a degree are affected by the selective pressures of evolution. Individuals tend to choose partners in marriage who are in the same intelligence range[8], which acts as an assortative mating mechanism. This kind of mating can reduce variation within a species and develop trends towards certain phenotypes, possibly creating ‘cognitive castes’[9]. In our culture, the phenotype is an increase in IQ over successive generations.
Much of the excitement generated by genetic intelligence studies hasn’t all been positive. Some research in this field is highly prone to incorrect interpretation and subject to racism. One of the most controversial books on this topic was The Bell Curve, which caused much uproar by reasoning that the over representation of minorities in the lower ranges of IQ scores meant that they were of an inferior stock of intelligence. Many adversaries of this view pointedly note that the language of the test is not usually the first language of the test-takers, therefore, the test-takers usually score lower than they would have. These debaters also point out that because of racial inequality, these people usually live in lower socioeconomic classes and are not afforded the same privileges as others and therefore do not have as many chances to express their intelligence.
The research of human intelligence will progress onward for years to come. as genetics continues to grow and develop, so will our understanding of our own aptitude; understanding of particular genes and the interactions of genes with their environments. And while much controversy will still surround the “nature/nurture” debate, the appearance of false positives reported, and the tracking of evolutionary changes will continue in this field, eventually we will uncover the secrets of the mechanics of human aptitude and intelligence.
[1] Oxford Dictionary of Biology, Oxford Press, Cambridge 6th edition, 2008
[2] OMIM 603783; Devlin et al.
[3] Devlin, B.; Fienberg, Steven; Resnick, Daniel; Roeder, Kathryn; Intelligence, Genes and Success; 1997 ( Copernicus Publishing, NYC) pgs.56-57
[4] Ridley, Matt. Genome; HarperCollins Press, NYC (1999) pg. 85
[5] University of California - Irvine. "Brain Network Related To Intelligence Identified." ScienceDaily 19 September 2007. 31 October 2008
[6] Duke University. "Breastfeeding Boosts IQ in Infants With 'Helpful' Genetic Variant." ScienceDaily 6 November 2007. 31 October 2008
[7] Haslinger, Kiryn.," A Jewish gene for intelligence?" Scientific American September 2005
[8] Vernon, Philip E., Intelligence: Heredity and Environment; 1979 (W.H. Freeman and Company, San Francisco) pg. 184
[9] Devlin et al. Pg 59
