There is no scientific evidence for a single intelligence gene: intelligence is too complex a phenomenon (some even argue whether such a thing as “intelligence”) exists) to be controlled by a single gene, or even a simple combination of a few genes. However, there is a lot of evidence demonstrating that intelligence is, to a large degree, hereditary and thus highly influenced by genes.
== What is intelligence? ==
Let us start with a look at the concept of intelligence: the answer to the question asking whether intelligence it’s controlled by genes or not depends to a large extent on what is meant by intelligence. And it can mean quite different things, depending on who’s speaking, and when. In psychology, there are many competing theories of intelligence, and many statements stating that is not possible to define it at all.
A 1994 definition defines intelligence as “a very general mental capability that, among other things, involves the ability to reason, plan, solve problems, think abstractly, comprehend complex ideas, learn quickly and learn from experience.” A dictionary claims that intelligence is “the ability to learn or understand or to deal with new or trying situations; the ability to apply knowledge to manipulate one’s environment or to think abstractly as measured by objective criteria”.
In real life it’s very clear that what is perceived as intelligence differs widely by situation, and even more widely by culture, and thus the same person might be seen as intelligent in one situation and pretty average in another. Dyslectic individuals might be seen as very intelligent in a situation that doesn’t require reading and writing, or in non-literate cultures, but will be commonly perceived as less intelligent in a literacy focused environment like school.
Knowledge, while clearly not hereditary, affects the apparent intelligence. If intelligence is the ability to apply knowledge, there must be knowledge to apply, and however high their potential ability to function and reason, a person placed in an alien social or physical environment will appear as less intelligent than somebody that lived there all their life.
With all these caveats, though, one can still intuitively sense that there is *something* – not easy to define and grasp, but nevertheless quite real – that underlies people’s cognitive performance and ability to learn and apply knowledge. It becomes most obvious when observing people of the same ages and sexes, coming from the same social background, having gone through the same educational process. Some of them perform better than others. Some are, quite clearly, not just more educated or more confident, or more familiar with the rules and facts, but are quicker, more flexible, better able to select from available options, make better use of the knowledge they have and identify the knowledge they need to gain. They are, quite clearly, more intelligent. Comparisons between people from different social groups, and even more so, between different cultures are much more risky and can lead to plainly ridiculous results such as those that claim that some sub-Saharan African peoples are less intelligent, on average, than moderately-retarded individuals in the West, a claim preposterous considering the ability of the people in questions to survive in their environment, manage complex kinship network, create art and religion.
== Is intelligence hereditary? ==
There is no argument that hereditary factors (including genes) can affect intelligence: from Down syndrome (which is congenital rather than hereditary, and chromosomal rather than genetic aberration) to fragile-X syndrome (a single-gene disorder). More than half of cases of severe mental retardation can be attributed to genetic causes. Without a doubt, a single gene metabolic disorder (e.g. PKU) can affect the development of the brain and nervous system enough to make it impossible for normal intelligence to develop.
However, there is no evidence for a single gene controlling intelligence (even if we only think of so called “general factor” intelligence and not any of the specific ones) and normal intelligence is considered to be a polygenic trait, influenced by an interaction of many individual genes. Some researchers estimate that close to half of all genes possibly contribute to the combined intelligence.
The interesting – and contentious – question is how much of the variability in the intelligence of normal individuals can be attributed to hereditary factors.
The most common method used to establish hereditability of any characteristics is using twin studies, and particularly studies of identical (monozygotic) twins separated at birth and reared apart. Studies of other siblings, parents and natural children, adoptive families and fraternal twins are also used.
Hereditability is defined as proportion of variation of a given characteristics that can be attributed to hereditary factor. This varies form 0 (all variation is due to environment) to 1 (all variation is hereditary). Estimates of the hereditability of intelligence vary from relatively low 0.4 to as high as 0.8, and it’s recognized that these scores are higher for adults than for children (more of the variability in children’s intelligence is caused by environment than variability of adults’ intelligence). A commonly quoted average is 0.75 – which means that three quarters of variability in adult intelligence is due to hereditary factors.
Identical twins have the same genotype and if intelligence was completely determined by hereditary factors, identical twins reared apart would have the same intelligence. Any difference in the intelligence between identical twins must be caused by environmental influences. Identical twins reared apart have very similar IQ scores (0.86 correlation) while fraternal twins reared together (who share, on average, only 50% of the genotype, but significantly more of the environmental influences, and who show 0.6 correlation). Adoptive siblings have uncorrelated intelligence scores.
== Environmental influences on hereditary intelligence ==
It’s worth noting that even if the large proportion of the factor described as intelligence is hereditary, it doesn’t mean that it cannot be influenced by environment to make a large practical difference. It’s probably more accurate to think that a certain potential for intelligence is hereditary, but that the actual, manifest level of intelligence is a result of a complex interplay of genetic and environmental factors.
Firstly, many environmental influences can combine to prevent an individual from developing their full genetic potential: from intra-uterine or early childhood malnutrition to diseases that affect development, to any trauma that might be caused by injury to the most obvious influences of education and exposure to varied stimuli.
Similarly, education and training can be used to vastly improve cognitive functioning of an individual with an average intelligence, compensating with skill for a lack (or not-outstanding) levels of mental ability. As a well known American novelist once said, you can’t make a pig into a racing horse, but you can make it into a very fast pig.