How many times have we been told that we look like our parents? The comparisons can even be obnoxious, but there are many times when we think we are a living reflection of our father or mother.
For many years, it has been a question of seeing how genetics influence human behavior, of making a child behave like his father at his age, or of trying to figure out how sometimes, when two twins are separated and raised by different families, even if they do not know each other, behave. very similar.
The environment influences the way everyone is, but genetics is something that is there and undoubtedly weighs its weight. however, How to determine to what extent it exerts its force?
In this article, we will try to explain 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 or cannot be inherited.
Heritability: basic definition
Heritability is an index or statistical parameter that estimates the proportion of phenotype variance in a population.That is, the psychological and physical traits that are manifested in individuals, attributable to genetic variation, that is, to the different genes that each person in the population is studied with.
The degree of heritability is expressed as a percentage or a value from 0 to 1, ranging from the absolute absence of hereditary weight of the phenotypic trait to the total heritability of itself, indicating this total heritability that the influence of the environment is zero.
Is it really possible to estimate what is the cause of the environment and what is the cause of genetics?
In recent years, and in particular thanks to better research in the field of epigenetics, it has been possible to understand the importance of environment and genes in terms of behavior and physical attributes of a person. However, many have defended the idea that the environment and genetics influence in the same way, at a percentage of 50% each.
Based on a hypothetical example linked to the definition of heritability given in the previous section, What would it mean that alcoholism in Spain has 33% heritability? Does this mean that 33% of alcoholism can be explained in genetic terms and the remaining 67% in environmental terms? Will 33% of the offspring of an alcoholic be alcoholic? Does the son of an alcoholic have a 33% chance that he is too? Does the population have a 33% risk of becoming an alcoholic?
None of the above questions would give a resounding “yes”. The term heritability actually refers to a population as a whole, based on data obtained by studying a group of people considered representative of it. For this reason, it is not possible to know to what extent truly genetic and environmental is behind a phenotypic trait in a particular individual. Also, it should be noted that when data is obtained from a sample, in turn, from a specific population.
That is, to return to the previous example, after studying 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 must conduct studies in these countries and take into account any changes in the environment.
How genetics really influence a personality type or disorder
Personality is a very complex aspect. Everyone sees similarities in the way they behave and in the way that one of their parents or loved ones did. However, reducing the whole broad term personality implies to a small set of genes is what has been called genetic reduction, a somewhat misleading belief.
This idea argues that personality or mental disorders are inherited, being influenced by presenting the genotype with one or two genes. In human behavior, in addition to the environmental factors that may arise, there are several genes involved, which may or may not all have inherited from one or both parents.
Aspects such as skin tone or eye color are hereditary, as one or a small group of genes explaining these characteristics has been identified. On the other hand, for the personality, understood as a set of psychological traits, the thing is more complicated.
Today, and after the conclusions of the Human Genome Project in 2003, we know that not all genes are manifested and that not all are at the origin of a specific trait.
Since the concept of heritability was formulated and also since it was intended to determine the influences of genes on human characteristics and behaviors, different types of studies have been carried out.
The simplest were made with animals. In these, by selectively breeding animals, especially dogs, an attempt has been made to identify genetically determined traits. By crossing related endogamous individuals, such as siblings, over several generations, it has been possible to generate individuals with virtually identical genotypes. The idea behind this is that the differences found in animals that have almost the same genes are believed to be due to environmental factors.
however, the studies that provided the most data on our species were those in which the subjects were people. It makes sense to think that the people who will share the most 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 humans, proposed by Francis Galton, were family studies, studies of twins and studies of adoption, being particularly interesting those of twins which we will explain more clearly in this section.
In the case of families, there are both similarities in physical and behavioral characteristics among their members. It takes into account that they not only share genetics, but also the same environment. Among these members, there may be close to 50% inbreeding in the event that they are first-class relatives, such as between siblings and with parents. This same percentage of inbreeding is also among non-identical, i.e. dicygotic twins, whose genetic relationship between them is said to be essentially the same as that of two brothers born in different years.
However, this inbreeding rises to 100% in the case of identical or monozygotic twins. In these cases, they share the same genome and the same sex. Thanks to the fact that, speaking clearly and Catalan, these twins are a clone of the other, it is logical to think that any psychological difference is due to an environmental factor which one of the two may have witnessed while the other does not have .
The studies between identical twins acquire a great interest when they are carried out with separated people and raised by different families. Based on this, if behavioral similarities are found, it can be inferred that the shared behaviors will be the result of a genetic origin. In the event that they were bred together, it is not entirely possible to know to what extent their behavior is a product of genetics or a genetic interaction by the environment.
Several studies have examined how differences in behavior occur between twins, whether they are raised in the same environment or in separate families. Here are some of the most classic and important, the results setting a precedent in the study of the genetic-environment relationship.
One of the most famous is the Minnesota study of twins raised apart or Misra, launched in 1979 by David Thoreson Lykken and continued by Thomas J. Bouchard. Their sample consists of adult twins who were raised separately and taken to multiple countries. It’s really interesting, because data of all kinds has been collected: physiological, anthropometric, psychological, personality, common interests … In the Misra, the CI has been approached, obtaining a percentage of heritability between 70 and 76 %.
Another study that addressed the psychological aspects of twins raised separately is the Swedish Adoption / Twins Study in Aging (SATSA). The principal investigator was Nancy Pedersen, the objective was to study the origins of the variability of longitudinal aging. During the study, a questionnaire on different aspects of health and personality was used on all twins in Sweden, or about 13,000 couples, half dizygotic and half monozygous.
In the case of the Nordic study, very interesting data was obtained in terms of intelligence, because in this case they took into account its heritability depending on the degree of intelligence. Pedersen had a heritability of 0.77 among the smartest twins, and a slightly lower, 0.73, among the less intelligent. In terms of personality, monozygotic twins had a correlation of 0.51 and dicygotic twins 0.21.
From these studies and many others in which the same objective has been approached in a very similar way, the following can be concluded. During childhood, genetic factors appear to influence intelligence scores differently. Understood the IC in its broadest view, its genetic influence is the most important, being close to 50%. If, on the other hand, this construction collapses in its subdivisions, such as verbal and spatial abilities, processing speed … it drops slightly, close to 47%.
Despite these results, it should be noted that many twin studies make methodological errors that contribute to inflating heritability values. One, already mentioned above, is the fact that sometimes, out of ignorance of one’s own family, their identical twins are not. There are cases of dizygotic twins that are so similar that they are mistaken for monozygotes.
Another mistake is to put genetics aside and attribute the similarity of twins in terms of behavior because their parents treat them the same. There aren’t a few families who put the same clothes on, buy them the same toys, or do the same with both because they are the same and should taste the same.
On this point, research, as in the case of Loehlin and Nichols in 1979, has observed that parental efforts to treat their twins equally or not do not appear to be a very environmental factor in terms of behavior.
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