There many arguments that help us to understand the reasons why this subject remains controversial; they are derived from both the intrinsic complexity of intelligence and the different initial premises with which the studies are conducted.
In any case, the Flynn effect shows and increase in intelligent quotients in different countries. The results of the Flynn effect are accepted.
The problem is with the reasons, causes and interpretation of the facts presented by the Flynn effect.
Below, the most common views are mentioned.
This view of the concept of intelligence is somewhat negative.
Francis Galton (1822-1911), cousin of Charles Darwin, indicated the necessity of using statistical methods to verify theories; thus, in his important work Natural Inheritance (1889) he introduced the concept of line of regression from a study comparing the statures of parents and children.
In the descriptive analysis of Galton's data, tall parents were observed to have tall children (but not so tall on average) and that short parents had short children (but not so short on average). This produced what he denominated a regression to the mean.
Perhaps the phenomena in which the famous regression to the mean takes place can be explained in greater detail with a multifactor analysis approach.
Richard J. Herrnstein and Charles Murray make many references to studies on human intelligence in their book The Bell Curve with different conclusions about the genetic influence in intelligence, including the famous Flynn effect. For the development of their ideas they assume an approximate correlation of 0,5 remaining in between those in favor of genetic influence and those in favor of environmental influence.
There is no general agreement on the stability of these capacities throughout life, although it seems that it is accepted that the average environmental influence is greater in early ages, followed by a decreasing influence until maturity. The latter is contrary to what would be expected.
In order to try to resolve the controversy on genetic and environmental influences in intelligence, numerous works have been performed, most of which they have been based on the study of identical twins.
The studies with identical twins have many advantages as they avoid some elements that could cause differences in intelligence. Even the Flynn effect is eliminated as the effect would operate in both identical twins.
Identical twins have a correlation of up to 0.87 as far as intelligence is concerned; in non-twin siblings correlation oscillates around 0.55. This data comprise of an experience of Jensen in 1972, which led to his basic conclusion that 80% of the variance in a population, related to the figures of the intellectual quotient (IQ), can be explained by inherited factors.
Logically, if this conclusion were correct we would have to assume that intelligence has basically a hereditary nature, although it is not predetermined because there are factors like genetic combination in accordance with the laws of Mendel.
At this point, it is worthwhile to remember the concept of hereditariness in a strict sense that is established by the relation between the observed and the expected correlations. In those cases in which the expected correlation is less than the unit, an upward correction of the observed correlation will be produced for the calculation of the degree of hereditariness.
Studies of great statistical complexity have also been made to try to resolve the controversy. Two of them drew my interest. I believe that one is eminently theoretical and the other practical.
The article Heritability Estimates Versus Large Environmental Effects: The IQ Paradox Resolved by William T. Dickens and James R. Flynn (Author of the Flynn effect), affirms to have solved the problem by means of the introduction of variables with temporary feedback. In my opinion, it is not surprising that, if we are already working with strongly correlated variables and we add a certain feedback, high statistical results can be reached.
On the other hand, this article tries to explain the observed Flynn effect or gain in IQ throughout different generations, specifically, the 20 point increase that occurred between 1952 and 1982 in some countries.
The other study, discriminating pre- and postnatal factors, from the Medical School of the University of Pittsburgh, reaches the conclusion that the prenatal maternal environment exerts a powerful influence on intelligence.