How many times have we been told that we look like our parents? Comparisons may even be hateful, but it is not rare for us to think that we are a living reflection of our father or mother.

For many years it has been tried to see how genetics influence human behavior, making a child behave like his or her father at his or 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 similarly.

The environment influences everyone’s way of being, but genetics is something that is there and exerts its weight without any doubt. However, 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.

Heredity: Basic definition

Heritability is an index or statistical parameter that estimates the proportion of the variance in the phenotype in a population , that is, the psychological and physical features that become manifest in individuals, attributable to genetic variation, that is, the different genes that each person in the population studied has.

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 the total heritability of the same, 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 improved 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 behaviour and physical attributes. However, many have advocated the idea that environment and genetics influence each other in the same way, at a rate of 50% each.

Based on 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 33% heritability? 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 ending up as an alcoholic?

None of the above questions would answer with a clear ‘yes’ . 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. Because of this, it is not possible to know to what extent genetics and environment are actually behind a phenotypic trait in a particular individual. Furthermore, it should be noted that when data are obtained from a sample this part, in turn, is from a particular 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 happens in other parts of the world, such as Saudi Arabia or Russia. To do so, 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 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 one of their parents or close relatives did it. However, reducing the whole broad term of personality to a small set of genes is what has been called genetic reductionism, a belief that is somewhat fallacious.

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

Aspects such as skin tone or eye colour are inherited, because one or a small group of genes have been identified to explain these characteristics. For personality, on the other hand, understood as a set of psychological traits, things are more complicated.

Nowadays, 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 particular trait.

Twin studies

Since the concept of heritability was formulated and also since the attempt to determine the influences of genes on human characteristics and behaviour, different types of studies have been carried out.

The simplest have been those made with animals. In these, by selectively breeding animals, especially dogs, we have tried 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 is that the differences found in animals that have almost the same genes are due to environmental factors.

However, the studies that have allowed us to obtain more data on our species are those in which the subjects were people . It is logical to think that the people who will share more 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 methods of research on heritability in human beings, proposed by Francis Galton, were 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, there are both similarities in physical and behavioural characteristics among their members. This takes into account the fact that they not only share genetics, but also the same environment. Among these members there may be a consanguinity of close to 50% in case 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 twins, that in essence the genetic relationship between them would be the same as that of two siblings born in different years.

However, this inbreeding is 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 difference is due to some environmental factor that one of the two has been able to witness while the other has not.

Identical twin studies are of great interest when done with those who have been separated and raised by different families. On this basis, if behavioural similarities are found, it can be deduced that the shared behaviours will be the result of a genetic origin. In case they have been raised together, it is not really possible to know to what extent their behaviour is a product of genetic or environmental genetic interaction.

Several studies have addressed how behavioral differences occur between twins, whether raised in the same environment or in separate families. The following are some of the more classic and important ones, whose results set a precedent in the study of the genetic-environmental 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. Its sample is composed of adult twins that were raised separately and has been carried out in multiple countries. It is really interesting, given that data of all kinds have been collected: physiological, anthropometric, psychological, personality, common interests… In MISRA the IQ has been addressed, obtaining a percentage of heritability of between 70-76%.


Another study that addressed psychological issues among separately raised twins is The Swedish Adoption/Twin Study of Aging (SATSA). The lead researcher was Nancy Pedersen, whose goal was to study the origins of variability in aging longitudinally. During the study a questionnaire on different health and personality aspects was used to 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 they took into account its heritability according to 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 ones. In terms of personality, monozygotic twins had a correlation of 0.51 and dizygotic twins 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 seem to differentially influence intelligence scores. Understanding IQ in its broadest vision, its genetic influence is the greatest, being close to 50% . If, on the other hand, this construct is broken down into its subdivisions, such as verbal and spatial abilities, processing speed… drops slightly, about 47%.

Despite these results, it should be noted that many of the twin studies make some methodological errors that contribute to inflating heritability values. One, already mentioned above, is the fact that sometimes, due to the ignorance of the family itself, identical twins turn out not to be. There are cases of dizygotic twins that are so similar that they are confused with monozygotic twins.

Another flaw is to leave genetics aside and attribute the similarity of the twins’ behavior to the fact that their parents treat them the same way. Many families put the same clothes on them, buy the same toys for them, or do the same thing to both of them because they are the same and should have the same tastes.

On this point, research, as in the case of Loehlin and Nichols in 1979, has observed that parents’ efforts to treat their twins equally or otherwise differently does not appear to be a strong environmental factor in their children’s behaviour.

Bibliographic references:

  • Andrés Pueyo, A. (1997). Heredity and the means of determining individual differences. In Manual de psicología diferencial (chap. 11). Madrid: McGraw-Hill.
  • Eysenck, H. J. (1991). The confrontation about intelligence: inheritance-environment? Madrid: Pirámide.
  • Lewontin, R., Rose, S., and 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 behaviour. Barcelona: Ariel.
  • Wright, W. (2000). This is how we are born: genes, behavior and personality. Madrid: Taurus.
  • Yela, M. (1996). Environment, heritage and behavior. Psicothema, 8, 187-228.