Everyone has once wondered if people are smarter because of their genetic inheritance or because of the environmental influences they receive, such as the quality of nutrition or the education provided by their parents. In recent years, behavioural genetics has managed to respond in a detailed way to this historical question.

Research in the field of differential psychology reveals that both genes and environment have a very significant weight in the determination of IQ, the classic measure of intelligence. However, the relevance of heredity seems to be slightly higher than that of the environment.

How do you define intelligence?

The construct “intelligence” is difficult to delimit, since multiple meanings have been attributed to it both in lay language and by the scientific community. It is a complex capacity that includes the ability to learn new information, to apply different types of reasoning and to solve problems, among many others.

A special definition is the one made from the operational approach. This perspective proposes that intelligence should be defined as “that which is measured by the IQ tests” , instruments that have been shown to be moderately useful in predicting aspects such as work performance and socioeconomic status.

However, intelligence is a very broad attribute that does not exist only in human beings. It has been defined by many authors as the capacity to behave in an adaptive manner before complex situations in order to achieve a goal; in this type of definition, the conception of intelligence as a global and stable factor stands out.

Relationship between genetics and intelligence

From the field of behavioural genetics, which analyses individual differences in behavioural aspects (such as intelligence) using genetic methods, the heritability coefficient of the IQ is estimated to be between 0.40 and 0.70. This means that approximately half of the variability is explained by hereditary factors .

From reviews in studies of this type, Antonio Andrés Pueyo concludes that approximately 50% of the variance in intelligence is explained by genetic causes, while the other 50% is due to different environmental factors and random errors of measurement.

In general, older studies have found a greater weight of genetic inheritance on intelligence than recent research. Furthermore, it seems that the heritability coefficient is higher in cases where the IQ is very high (over 125) or very low (under 75).

As for the different factors that make up intelligence, some studies have found that verbal skills are inherited to a greater extent than manipulative ones. The weight of genetics in verbal IQ increases with age ; the same happens with other components of intelligence, although not as notably.

On the other hand, the fluid intelligence described by Raymond B. Cattell, a construct similar to the global factor (“g”) originally used by the pioneer Charles Spearman, is more influenced by genetic inheritance than crystallized intelligence. While the former is associated with reasoning and new problem solving, the latter refers to accumulated knowledge

Influence of brain structure and processes

Different authors have pointed out the relevance of the physiological processes of the central nervous system on intelligence. In this sense, structures and functions such as the frontal lobes, the density of grey matter (composed of neuronal bodies, amyelinic dendrites and glia) in the brain or the metabolic rate of glucose are involved.

Thus, Vernon wrote that the differences found in the IQ tests reflect a greater speed and efficiency in the transmission of nerve impulses, while according to Eysenck the most important thing is the number of errors in these connections: if there are fewer transmission errors the brain will consume less glucose , reducing energy effort.

Other studies have linked intelligence measurements to blood flow and neurochemical activity in the frontal lobes, as well as to the density of grey matter. All these morphological and functional characteristics are inherited to a significant degree, since they depend on the expression of certain genes.

Environmental factors affecting IC

Intelligence depends largely on the environment. In this sense, a large number of factors are relevant, among which the most important is access to quality nutrition, education and health care that allows the greatest possible development of the biological potential of each person’s brain.

In many cases it is extremely difficult to determine what proportion of behavioural variability can be attributed to heredity and what proportion to environment, particularly when we are talking about influences relating to the immediate family environment. There is also a constant interaction between genetics and the environment.

According to Andrés Pueyo, environmental factors explain almost half of the variance in intelligence, a weight very similar to that of genes. Within the 50% of variability that is not justified by heredity he attributes 30% to common or inter-family variance and 10% to the non-shared environment . The error variance weights another 10% for this author.

Thus, non-shared environmental influences, which differ between people raised in the same family, seem to be more relevant in determining intelligence than the shared environment, although the weight of the latter is sufficiently high to be taken into consideration.