Genes for Mental Retardation and Intelligence
Intelligence is, in itself, a somewhat controversial topic. But even more controversial is the association between genetics and intelligence. For years, controversy has developed on the basis of a set of data relatively limited studies that are heritability, twin studies, adoption studies etc. Overall, these studies show that people are more similar genetically, plus their intelligence scores are close. This leads to say that intelligence is heritable in both percent and therefore genetic influences on intelligence. The problem is that it is incomplete as a basis for argument. Once you said that there are genetic influences on intelligence, the immediate question that arises is which. And how they manifest themselves? The results of recent research in molecular genetics, both brain development as the development of cognitive functions in children, can give a little substance to the idea that there is a genetic contribution to intelligence .
When we want to measure the intelligence of someone, typically, he administered tests that are quite different from each other and which are supposed to measure different aspects of cognitive functioning of the individual. Wechsler, the most widely used, contains a dozen tests, such as an arithmetic test, a test of verbal memory to remember where he figures, a test where you have to complete images etc.. And then all these tests are supposed to measure cognitive functions such as those shown in the diagram. This is not an exhaustive list.
The processing speed of the information would be a possible component. Similarly, attention span, memory capacity long term, short term, verbal skills, numeracy skills, visual-spatial abilities are all cognitive functions which may be thought that these are components that contribute to intelligence.
The great discovery, which dates already now the first half of the twentieth century is that the scores of people in all these tests are heavily correlated: when a person is good to test, it has likely to be good in the other tests, and vice versa. This has led to a theoretical formulation of this statistical fact: namely, that there is a factor underlying single, which explains most of the variations in these components. This is what we will call the g-factor, for “general intelligence”.
It is primarily a statistical reality. That is to say, when you do a factor analysis scores in all tests yields a single factor that explained most of the variance. Nevertheless, there are several ways to interpret this. Does this factor g corresponds to a true cognitive entity, in terms of information processing, or biological property representing a brain? Or has the statistical a reality?