Heritability: what is it and how does it affect our behavior?

How many times have we been told that we look like our parents? Comparisons can be even odious, but it is not infrequent that we believe that we are a living reflection of our father or mother.

For many years it has been tried to see how genetics influences human behavior, causing a son to behave like her father at her age or trying to understand how sometimes when two twins are separated and raised by different families, despite not knowing each other, they behave very similar.

The environment influences the way of being of each one, but genetics is something that is there and that exerts its weight without any doubt. Nevertheless, How is it possible to determine to what extent it exerts its force?

In this article we will try to address what is meant by heritability and some of the research that has been carried out to try to understand how personality, cognitive abilities and behavior can be inherited or not.

Heritability: Basic Definition

Heritability is an index or statistical parameter that estimates the proportion of variance in phenotype in a population

, that is, the psychological and physical traits that come to manifest in individuals, attributable to the genetic variation, that is, the different genes that each person in the population has studied.

The degree of heritability is expressed as a percentage or value from 0 to 1, ranging from the most absolute absence of hereditary weight from the phenotypic character to its total heritability, indicating this total heritability that the influence of the environment is null.

Is it really possible to estimate what is due to the environment and what is due to genetics?

In recent years and, above all, thanks to better research in the field of epigenetics, it has been possible to understand how important the environment and genes are in terms of behavior and physical attributes of a person. However, there are not a few who have defended the idea that the environment and genetics influence in the same way, in a percentage of 50% each.

Starting from a hypothetical example and related to the definition of heritability given in the previous section, What would it mean that alcoholism in Spain has a heritability of 33%? Does it mean that 33% of alcoholism can be explained in genetic terms and the remaining 67% in environmental terms? Will 33% of the descendants of an alcoholic be alcoholics? Does the son of an alcoholic have a 33% chance of him being one too? Does the population have a 33% risk of ending up being an alcoholic?

None of the above questions would give a resounding 'yes' as an answer.. Actually the term heritability refers to a population as a whole, from the data obtained by studying a group of people that is considered representative of it. Due to this, it is not possible to know to what extent genetics and environment are really behind a phenotypic trait in a specific individual. In addition, it should be noted that when data are obtained from a sample this part, in turn, from a specific population.

In other words, going back to the previous example, having studied alcoholism in the Spanish population, we know the percentage of heritability of this trait in people who share the same environment or live in the same region, in this case Spain. We cannot know from this data what is happening in other parts of the world, such as Saudi Arabia or Russia. For this we will have to carry out studies in those countries and take into account the changes in the environment that may occur.

To what degree genetics actually influence a personality type or disorder

Personality is a very complex aspect. Everyone sees similarities in the way they behave and how one of their parents or a close relative did. However, reducing the whole broad term that personality implies to a small set of genes is what has been called genetic reduction, a belief that is somewhat fallacious.

This idea holds that personality or mental disorders they are heritable, being influenced by the presence of one or two genes in the genotype. In people's behavior, in addition to the environmental factors that may occur, there are multiple genes involved, all of which may or may not have been inherited from one of the two parents or from both of them.

Aspects such as skin tone or eye color are inheritable, because one or a small group of genes that explain these characteristics have been identified. On the other hand, for the personality, understood as a set of psychological traits, things are more complicated.

Today, and following the conclusions of the Human Genome Project in 2003It is known that not all genes are manifested, nor is each of them behind a specific trait.

Twin studies

Since the concept of heritability was formulated and also since it was tried to determine which were the influences of genes on human characteristics and behaviors, different types of studies.

The simplest have been those made with animals. In these, by selectively breeding animals, especially dogs, an attempt has been made to identify genetically determined traits. By inbreeding related individuals, such as brothers and sisters, over several generations it has been possible to generate individuals with practically identical genotypes. The idea of ​​this is that the differences found in animals that have almost the same genes are due to environmental factors.

Nevertheless, the studies that have allowed the most data to be obtained on our species are those in which the subjects were people. It is logical to think that the people who will share the most genes are those who are part of the same family, but there should be more relationships between those people who are identical twins.

Thus, the three research methods on heritability in humans, proposed by Francis Galton, were the studies in families, studies of twins and studies of adoptions, being especially interesting those of twins that we are going to expose with more clarity in this section.

In the case of families, between their members there are both similarities in physical and behavioral characteristics. The fact that they not only share genetics but also the same environment is taken into account. Among these members there may be a consanguinity close to 50% if they are first-order relatives, such as between siblings and with the parents. This same percentage of consanguinity is also found among non-identical twins, that is, dizygotic, that in essence the genetic relationship between them would be the same as that of two brothers born in different years.

However, this consanguinity rises to 100% in the case of identical or monozygotic twins. In these cases they share the same genome, in addition to the same sex. Thanks to the fact that, simply speaking, these twins are a clone of the other, it is logical to think that any psychological difference is due to some environmental factor that one of the two has been able to witness while the another does not.

Identical twin studies are of great interest when done with those who have been separated and raised by different families. Based on this, if behavioral similarities are found, it can be deduced that the shared behaviors will be the result of a genetic origin. In case they were raised together, it is really not entirely possible to know to what extent their behavior is the product of genetics or a genetic interaction by environment.

Several studies have addressed how behavioral differences occur between twins, whether they are raised in the same environment or in separate families. Some of the most classic and important are explained below, the results of which set a precedent in the study of the genetic-environment relationship.

One of the most famous is the Minnesota Study of Twins Reared Apart or MISRA, started in 1979 by David Thoreson Lykken and continued by Thomas J. Bouchard. His sample is made up of adult twins who were raised separately and has been conducted in multiple countries. It is really interesting, given that all kinds of data have been collected: physiological, anthropometric, psychological, from personality, common interests... IQ has been addressed in the MISRA, obtaining a heritability percentage of between 70-76%.

Intelligence

Another study that addressed psychological aspects among twins raised separately is The Swedish Adoption / Twin Study of Aging (SATSA). The principal investigator was Nancy Pedersen, whose objective was to study the origins of variability in aging longitudinally. During the study, a questionnaire on different aspects of health and personality was used for all twins in Sweden, about 13,000 couples, half dizygotic and half monozygotic.

In the case of the Nordic study, very interesting data were obtained with respect to intelligence, because in this case their heritability was taken into account based on the degree of intelligence. Pedersen obtained a heritability of 0.77 among the most intelligent twins, and a slightly lower one, 0.73, among the least intelligent. Regarding personality, monozygotic twins had a correlation of 0.51 and dizygotic twins 0.21.

From these studies and many others in which the same objective was approached in a very similar way, the following can be concluded. During childhood, genetic factors appear to influence intelligence scores differentially. Understanding IQ in its broadest vision, its genetic influence is the greatest, being close to 50%. If instead this construct is broken down into its subdivisions, such as verbal and spatial capacities, processing speed... it drops slightly, about 47%.

Despite these results, it should be noted that many of the twin studies make some methodological flaws that contribute to inflate heritability values. One, already commented previously, is the fact of ignoring that sometimes, due to ignorance of the family itself, their identical twins turn out that they are not. There are cases of dizygotic twins that look so alike that they are mistaken for monozygotic.

Another failure is to leave out genetics and attribute the similarity of the twins in terms of their behavior because their parents treat them in the same way. There are not a few families who put the same clothes on them, buy them the same toys or do the same with both because as they are the same they should have the same tastes.

On this point, research, such as Loehlin and Nichols in 1979, has observed that parents' efforts to Treating their twin children the same or, otherwise, differently does not seem to be an environmental factor of much weight in terms of the behavior of these.

Bibliographic references:

• Andrés Pueyo, A. (1997). Heredity and the environment in determining individual differences. In Manual of Differential Psychology (ch. 11). Madrid: McGraw-Hill.
• Eysenck, H. J. (1991). The confrontation on intelligence: inheritance-environment? Madrid: Pyramid.
• Lewontin, R., Rose, S., and Kamin, L. (2003). It is not in the genes. Racism, ideology and genetics. Barcelona: Critical Ed.
• Pinker, S. (2003). The clean sweep: the modern negotiation of human nature. Barcelona: Paidós.
• Plomin, R., DeFries, J. C., and McClean, G. AND. (2002). Genetics of behavior. Barcelona: Ariel.
• Wright, W. (2000). This is how we are born: genes, behavior and personality. Madrid: Taurus.
• Yela, M. (1996). Environment, inheritance and behavior. Psicothema, 8, 187-228.