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 even be hateful, but there are many times that we believe that we are a living reflection of our father or mother.
For many years we have tried to see how genetics influences human behavior, making a child behave like his father at his age or trying to understand how, sometimes, when two twins are separated and raised by different families, despite If they don’t know each other, they behave in a very similar way.
The environment influences each person’s way of being, but genetics is something that is there and that exerts its weight without a doubt. However, How is it possible to determine to what extent it exerts its force?
In this article we are going to 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 is a statistical index or parameter that estimates the proportion of variance in the phenotype in a population that is, the psychological and physical traits that manifest in individuals, attributable to genetic variation, that is, the different genes that each of the people in the studied population have.
The degree of heritability is expressed as a percentage or value from 0 to 1, going from the most absolute absence of hereditary weight of the phenotypic character to its total heritability, this total heritability indicating that the influence of the environment is null.
Is it really possible to estimate what is due to 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 the behavior and physical attributes of a person. However, there are many who have defended the idea that the environment and genetics influence in the same way, at 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 child of an alcoholic have a 33% chance of being one too? Does the population have a 33% risk of becoming an alcoholic?
None of the above questions would yield a resounding ‘yes’ answer Actually, the term heritability refers to a population as a whole, based on the data obtained by studying a group of people that is considered representative of it. Because of this, it is not possible to know to what extent genetics and environment are really behind a phenotypic trait in a specific individual. Furthermore, it should be noted that when data is obtained from a sample, it is, in turn, part of a specific population.
That is, returning 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. To do this we will have to carry out studies in those countries and take into account any changes in the environment that may occur.
To what degree does 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 entire broad term that is personality to a small set of genes is what has been called genetic reduction, a belief that is somewhat fallacious.
This idea maintains that personality or mental disorders are heritable, being influenced by having one or two genes in the genotype. In the behavior of people, 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 both.
Aspects such as skin tone or eye color are heritable, because one or a small group of genes have been identified that explain these characteristics. On the other hand, for personality, understood as a set of psychological traits, things are more complicated.
Today, and after the conclusions of the Human Genome Project in 2003, it is known that not all genes are manifested nor is each one of them behind a specific trait.
Twin studies
Since the concept of heritability was formulated and also since the attempt was made to determine the influences of genes on human characteristics and behaviors, different types of studies have been carried out.
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 endogamously crossing related individuals, such as brothers and sisters, over several generations it has been possible to generate individuals with practically identical genotypes. The idea is that the differences found in animals that have almost the same genes would be due to environmental factors.
However, The studies that have allowed us to obtain the most data about 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 human beings, proposed by Francis Galton, were family studies, twin studies and adoption studies, with twin studies being especially interesting, which we will explain more clearly in this section.
In the case of families, there are both similarities in physical and behavioral characteristics among its members. It takes into account the fact that they not only share genetics, but also the same environment. Among these members there may be a consanguinity close to 50% if they are first-order relatives, such as between siblings and parents. This same percentage of consanguinity is also found among non-identical twins, that is, dizygotic, who 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. Since, 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 other has not.
Studies among identical twins become of great interest when they are 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. If they were raised together, it is really not entirely possible to know to what extent their behavior is a product of genetics or a genetic-by-environment interaction.
Several studies have addressed how behavioral differences occur between twins, whether raised in the same environment or in separate families. Below, some of the most classic and important are explained, whose results 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. Their sample is comprised of adult twins who were raised separately and has been conducted in multiple countries. It is really interesting, given that all types of data have been collected: physiological, anthropometric, psychological, personality, common interests… In MISRA, IQ has been addressed, obtaining a heritability percentage of between 70-76%.
Another study that addressed psychological aspects between twins raised separately is The Swedish Adoption/Twin Study of Aging (SATSA). The main researcher 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 on all twins in Sweden, nearly 13,000 couples, half dizygotic and half monozygotic.
In the case of the Nordic study, very interesting data was obtained regarding intelligence, because in this case they took into account its heritability depending 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 less intelligent. Regarding personality, monozygotic twins had a correlation of 0.51 and dizygotic twins had a correlation of 0.21.
From these studies and many others in which the same objective was addressed in a very similar way, the following can be concluded. During childhood, genetic factors appear to differentially influence intelligence scores. Understanding IQ in its broadest vision, its genetic influence is the greatest, being close to 50% If, however, this construct is broken down into its subdivisions, such as verbal and spatial abilities, processing speed… it drops slightly, around 47%.
Despite these results, it should be noted that many twin studies make some methodological errors that contribute to inflating heritability values. One, already mentioned above, is the fact of ignoring that sometimes, due to the ignorance of the family itself, their identical twins turn out to be not identical. There are cases of dizygotic twins that look so similar that they are mistaken for monozygotic.
Another failure is to leave genetics aside and attribute the similarity of twins’ behavior to the fact that their parents treat them the same way. There are many families who put the same clothes on them, buy them the same toys or do the same thing with both of them because since they are the same they should have the same tastes.
Regarding this point, research, such as Loehlin and Nichols in 1979, has observed that parents’ efforts to treat their twin children the same or, otherwise, differently does not seem to be a very important environmental factor. weight in terms of their behavior.
