First of all, in order to make any further investigation into more complicated subjects, we have to define what intelligence is. This can be very complicated since not everyone agrees on what the definition of intelligence is. Many tests, like IQ tests, SAT, and GRE, are designed to measure someone's level of cognition. The scores for these tests have to be stable, valid, and they have to measure heritability. Stability means that the scores won't change throughout the lifespan of the person being tested. Validity means that the scores must have some predictive power. Heritability means that the scores will somewhat reveal the genetic causes for these scores.
IQ tests consist of smaller divisions that tests different areas of intelligence. For example: short term memory, vocabulary, math skills, and spatial rotations. These are some of the basic things that could be found on most intelligence tests. On the other hand, there are more controversial areas like reaction time and pitch recognition.
Since there is (for most people) a positive correlation in the different areas of the test, it is possible to put all the scores together to give one final number. Many people think that if someone is good at one of the subjects tested, then the scores will not be as high on other areas. But in reality, for most people, all these scores are positively correlated. All the scores put together can be defined as that person's g (general intelligence).
From studying the correlation in IQ scores of twins and normal siblings, it can be said that g is highly heritable. These studies were done on identical twins that grew up together and on identical twins that grew up apart, and the correlation was not very different. This shows that environment does not have much of an impact on intelligence.
Like other traits, for example height, intelligence depends on various alleles of a gene on the genome. What scientists are trying to do now is to identify the sites in the genome in which these "intelligence" genes can be found. At this point scientists still don’t know where these sites are located. But by how fast scientists are gaining more and more information on the human genome, it can be predicted that we will have this information within the next five to ten years.
Intelligence can be defined as an overall group of different factors. Most of the times if someone is good at one area, then this person will do well on all the other areas. This positive correlation shows that, although intelligence depends on many factors, it can still be measured by putting these different factors together and averaging them out.
The studies conducted on twins show that genes greatly affect intelligence. Since identical twins have identical genes, the only thing that could affect their scores would be outside environmental factors. The studies show that these factors don't affect the correlation as much as some people believe they do.
It is impressive that with all the research being done on this topic, scientists still haven't located the genes that are responsible for intelligence. Although we have a lot of information that we have gotten from studies like the twin studies, nothing can be certain until we locate and identify these genes in the genome.
- There is evidence that environment has a minor impact on intelligence. What are the environmental factors that affect intelligence?
- Why is it so much harder to locate the genes that are responsible for intelligence than it is for any other trait?