4.a.3 Identical or monozygotic twin studies
Several studies on intelligence or IQ inheritance among identical twin brothers, with identical genes, have shown correlations in the intelligence of 80% or close to this figure.
This argument regarding identical or monozygotic twin studies is definitive because it would not make much sense intelligence had a genetic component so high in some cases and others just the opposite.
Other question would be why it does not always appear like that. We believe Mendelian genetics is the cause of the apparent inconsistency.
An exciting aspect of monozygotic or identical twin studies on intelligence or IQ inheritance is that if the correlation of the twin's IQ with their respective parents were studied; the explained variance would be noticeably less than the mentioned percentage due to the genetic combination derived from Mendel's laws.
One of the authors that are most well-known for his IQ studies with twin brothers and his articles in favor of genetic influence on intelligence quotients is Arthur Jensen.
In their book, The Bell Curve; Charles Murray and Richard J. Herrnstein present an intermediate analysis regarding the heritability of IQ and intelligence; it brings together articles about twin brothers, adopted and standard siblings, and works with different, even contradictory conclusions, some of them with monozygotic and dizygotic or non-identical twins. It is a book on sociology and the consequences of education; it states there is a correlation between genetics and environmental influence that could generate pockets of populations with slower development.
4.a.4. Siblings and dizygotic or non-identical twin studies
From the measurements of intelligence carried out on siblings or dizygotic or non-identical twins, we can make two independent comments.
On the one hand, in statistical twin studies on IQ inheritance, if the observed correlation in one case is 40% and, taking into account Mendelian genetics, the expected is 50% of the real correlation; therefore, the degree of heritability will be the ratio 40% / 50% = 80%
To establish the expected correlation, in the case of siblings and dizygotic or non-identical twins, we would have to start with some theoretical hypotheses. It will not be the same if we knew the rules for determining the supposed dominant and recessive gene, or if different chromosomes are affecting the studied characteristic, the analysis would be quite complicated.
The second comment refers to the environmental circumstances; if they were important, it would be worth waiting for a greater resemblance between the IQ and intelligence quotients in siblings and dizygotic and non-identical twins than those observed. These circumstances are fundamentally equal within the same family, except if we pay disproportionate attention to the influence of a different math teacher or any other subject or; in such a case, we might find that the sum of all the parameters would be higher than the unit.
4.a.5. Studies among families: father, mother and one sibling
There is not very much of this type of work on intelligence inheritance, or it is less known; the results on the correlation of the IQ are fairly low. If the calculi had the correction regarding the observed and expected correlations, the results might not have been so low.
The genetic affinity in intelligence between parents and children will be, as a maximum, equal to that of siblings or dizygotic and non-identical twins.
The EDI Study – Evolution and Design of Intelligence belongs to this group. If the results are similar in additional studies, the debate over the heritability of intelligence could end, at least in its current scale. At the same time, it is possible that other profound debates are becoming more critical.
The obtained correlation was higher than 80% in many cases, reaching 96% and 99% in some of them.
The key to success doubled. On the one hand, the use of the Conditional Evolution of Life model; on the other hand, the EDI Study makes groups of the data set to compensate for variations due to the random component of the combination.
With the grouping methodology, it is not necessary to correct the rising correlation observed according to the expected correlation. There is the advantage that the expected correlation does not need to be known. Also, other possible variables of small intensity and random distribution that could affect intelligence or its manifestation and measurement will compensate automatically.
The multiple dimensions that the different groupings imply have allowed an analysis of sensitivity to be carried out in respect to the goal function and altering partial aspects of the model's structure and the parameters involved with a reasonable guarantee that the results are not due to somewhat random coincidences of the sample information.