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

We explain how heritability is determined and what applications it has.

How many times have we been told that we resemble our parents? Comparisons can even be odious, but there are many times when we believe that we are a spitting image of our father or mother.

For many years we have been trying to see how genetics influences human behavior, making a son behave like his father at his age or trying to understand how, sometimes, when two twins are separated and raised by different families, despite not knowing each other, they behave in a very similar way.

The environment influences the way each one is, but genetics is something that is there and that exerts its weight without any doubt. However, how is it possible to determine the extent to which 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 done 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, i.e., psychological traits.that is, the psychological and physical traits that become manifest in individuals, attributable to genetic variation, i.e., the different genes that each of the individuals in the population studied has.

The degree of heritability is expressed as a percentage or value from 0 to 1, going from the absolute absence of hereditary weight of the phenotypic character to its 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 a person's behavior and physical attributes. However, there are many who have defended the idea that environment and genetics have the same influence, at a percentage of 50% each.

Using a hypothetical example 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 also being an alcoholic? Does the population have a 33% risk of becoming alcoholics?

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

That is, returning to the previous example, since alcoholism has been studied 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 these data what is happening in other parts of the world, such as Saudi Arabia or Russia. For that we will have to conduct studies in those countries and take into account any changes in the environment that may occur.

To what degree does genetics really influence a personality type or disorder?

Personality is a very complex aspect. Everyone sees similarities in the way they behave and how a parent or close relative behaved. However, to reduce the whole broad term personality to a small set of genes is what has been called genetic reductionism, a somewhat fallacious belief.

This idea holds that personality or mental disorders are heritable, being influenced by the presence in the genotype of one or two genes. 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 or both parents.

Aspects such as skin tone or eye color are inheritable, 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 manifest themselves, nor is each one of them behind a specific trait.

Twin studies

Since the concept of heritability was formulated and also since it was intended to determine the influence of genes on human characteristics and behaviors, different types of studies have been carried out.

The simplest have been those done with animals. In these, by selectively breeding animals, especially dogs, attempts have been made to identify genetically determined traits. By inbreeding related individuals, such as brothers and sisters, individuals with virtually identical genotypes have been generated over several generations. The idea is that the differences found in animals with almost identical genes are due to environmental factors.

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

Thus, the three methods of research on heritability in human beings, proposed by Francis Galton, were family studies, twin studies and adoption studies, the twin studies being particularly interesting, which we will explain more clearly in this section.

In the case of families, among their members there are both similarities in physical and behavioral characteristics. This takes into account the fact that they share not only genetics, but also the same environment. Among these members there can be a consanguinity close to 50% in the case of first order relatives, such as between siblings and with the parents. This same percentage of consanguinity is also found among non-identical twins, i.e. dizygotic twins, which in essence the genetic relationship between them would be the same as that of two siblings 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, as well as the same sex. Because, simply put, these twins are a clone of each other, it is logical to think that any psychological differences are due to some environmental factor that one of the two has been able to witness while the other has not.

Studies among identical twins acquire great interest when done with those who have been separated and raised by different families. On this basis, if behavioral similarities are found, it can be deduced that the shared behaviors are the result of a genetic origin. In case they have been raised together, it is not really possible to know to what extent their behavior is a product of genetics or a genetic interaction by environment.

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

One of the most famous is the Minnesota Study of Twins Reared Apart or MISRA, initiated in 1979 by David Thoreson Lykken and continued by Thomas J. Bouchard. Its sample is composed of adult twins who were reared apart and has been conducted in multiple countries. It is really interesting, since all kinds 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%.

Intelligence

Another study that addressed psychological aspects between twins reared separately is The Swedish Adoption/Twin Study of Aging (SATSA). The principal investigator was Nancy Pedersen, whose aim 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 pairs, 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 they took into account its heritability according to the degree of intelligence. Pedersen obtained a heritability of 0.77 among the more intelligent twins, and a slightly lower one, 0.73, among the less intelligent ones. In terms of 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 seem to have a differential influence on intelligence scores. Understood in the broadest sense, the genetic influence on IQ is the greatest, being close to 50% of the total.. If, on the other hand, this construct is broken down into its subdivisions, such as verbal, spatial, processing speed .... slightly decreases, close to 47%.

Despite these results, it should be noted that many of the twin studies commit some methodological flaws that contribute to inflate heritability values. One, already mentioned above, is the fact of ignoring the fact that sometimes, due to ignorance of the family itself, their identical twins turn out not to be 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 in the same way. There are many families who put them in the same clothes, 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 that of Loehlin and Nichols in 1979, has observed that the efforts of parents to treat their twin children the same or, otherwise, differently does not seem to be an environmental factor of much weight in terms of their behavior.

Bibliographical references:

• Andrés Pueyo, A. (1997). Heredity and the environment in the determination of individual differences. In Manual de psicología diferencial (chap. 11). Madrid: McGraw-Hill.
• Eysenck, H. J. (1991). The intelligence confrontation: Inheritance-environment? Madrid: Pirámide.
• Lewontin, R., Rose, S., & Kamin, L. (2003). It's not in the genes. Racism, ideology and genetics. Barcelona: Ed. Critica.
• Pinker, S. (2003). The clean slate: the modern negotiation of human nature. Barcelona: Paidós.
• Plomin, R., DeFries, J. C., and McClean, G. E. (2002). Genetics of behavior. Barcelona: Ariel.
• Wright, W. (2000). Así nacemos: genes, conducta y personalidad. Madrid: Taurus.
• Yela, M. (1996). Environment, heredity and behavior. Psicothema, 8, 187-228.