Twin studies

We have been trying for some time to find out to what extent genetics and the environment influence different aspects of human personality, behavior and cognitive abilities. However, genetics and environment are not two aspects easily separable under laboratory conditions. In this article we will let you know about the twin studies.

We cannot separate a person from his family with the intention of controlling all the stimuli he receives, nor can we genetically modify him to see to what extent one or several genes are behind a particular trait.

Fortunately for scientists there are twins, especially those who, for various reasons, have been separated from each other. With the same genetics, identical twins allow you to control the hereditary factor and differentiate it more clearly from environmental influences.

Let’s look in more detail at what research or twin studies consist of , a type of natural study in which the ethical implications of separating a child from his family under laboratory conditions would not be violated.

More about twin studies

Twin investigations are tools whose sample is composed of twins, whether they are identical twins (monozygotic) or fraternal twins (dizygotic) .

Several of these studies were carried out in the last century and in the current one, with the aim of discovering what are the real influences of the environment and genetics on the different characteristics that the human being manifests, such as personality traits, cognitive abilities or incidence. of psychiatric disorders. They can be considered natural experiments, since nature provides us with individuals in which environmental variables can be isolated, having the same genes.

The origins of this type of study lie in the classic debate about what is more important in terms of people’s development, whether the environment or the environment, better known in English as ‘nature versus debate’. nourish ‘ . The first to propose the use of twins to study the heritability of traits, both physical and psychological, comes from Sir Francis Galton, best known for being a cousin of Charles Darwin and a pioneer in eugenic ideas.

Galton, in an 1875 document entitled The History of Twins , argues that nature, that is, what we now call genetics, is the predominant factor over the environment, espousing an innate idea of ​​both. Human personality behavior. Over time, in the 1920s, the methods proposed by Galton were improved.

In these early studies, the intention was to compare identical twins with fraternal twins, establishing the degree of heritability of certain degrees based on the differences observed in them. The idea behind this was that what was seen in the two identical twins had to be due to the genetic factor, especially if there was the unfortunate situation where these brothers had been raised separately.

What are worth for?

As we have already mentioned, there are two types of twins. There are monozygotic twins, popularly known as identical twins. These twins are the result of a fertilized egg dividing in the early stages of development, producing not one, but two fetuses from the same cell. So these types of twins are genetically identical, and to say they are the clone is, technically, true.

On the other side are dizygotic twins, also called in popular parlance as fraternal twins or twins. These twins originate because of the fertilization of two eggs, which implies that both fetuses develop at the same time in the mother’s womb, but are not genetically the same. The genetic relationship between these twins is the same as it would be with other siblings in the family , only that they were born at the same time. In fact, they can be of different sexes.

The utility of twin studies is especially related to monozygotic twins. This type of research tool makes it possible to control a factor that in other types of people would be impossible to control: genetics. That is, it is possible to compare two people who have the same “nature”, as Galton would say, to see to what extent their behavioral and cognitive differences are due to the environment.

These types of studies have contributed to making the nature of the vs. creation “becomes more and more moderate. It is now generally accepted that genetics and the environment have similar weights , but in the past the positions were quite polarized. While Galton defended innateness and that nature was everything, psychoanalysis by the hand of Sigmund Freud said the opposite. Psychoanalysts defended the idea that disorders like autism or schizophrenia were due to the way children were raised.

In short, the methodology of twin studies consists of calculating the correlations of those characteristics that are the object of study, detecting agreement or differences . Subsequently, these are compared between identical twins with those that are fraternal. Based on this, if the heritability factor of a particular trait is high, monozygotic twins would have to closely resemble that same trait. This genetic weight is especially important to measure in situations where the twins were raised separately.

An idea defended in relation to this type of research is the fact that it is possible to discover how the family environment, also called shared, is losing weight as the person grows. This phenomenon is especially noticeable in families where there are twins, whether fraternal or identical, because as they get older, it’s easier to see how different they are.

These differences may be due to several factors, which would be within the variable not shared or individual environment, such as a different group of friends, different teachers, favorite parents… However, it is still argued that in monozygotic twins, with high genetic similarity, both tend to look for the same components for their individual environments .

Famous research with twins

Below, we explain three of the most famous studies done with twins. They investigated the heritability of various traits, both physical and related to personality, mental disorders, and cognitive abilities .

1. The Minnesota Twin Study broke up (1979 to present)

Among the most important researchers in this study are Thomas J. Bouchard. This is one of the most famous surveys in the field, as its sample is made up of twins who were raised separately shortly after birth.

In this study, it was assumed that the similarities manifested by these twins must necessarily be due to their genetic basis . Among all the twins studied, some of them gathered a large number of coincidences especially drawing attention:

• Their names: James Lewis and James Springer.
• Married and divorced women called Linda.
• They married a Betty.
• Both received police training.
• They drank and smoked the same way.
• They bit their nails.
• His sons: James Alan Lewis and James Allan Springer.

And all these details are not the only ones. Coincidences of this kind are rare, but of course they can give bellows to those who think that everything is nature and not the environment.

The researchers found that about 70% of the IQ variation in the sample was due to a strong genetic component .

The study found that twins separated at birth and raised separately resembled twins raised in the same household, in aspects such as personality, gestures, social behavior, leisure and professional interests.

2. Swedish Adoption / Twin Study of Aging (SATSA) (80’s and 90’s)

Its principal investigator is Nancy Pedersen. Questionnaires were applied asking about 13,000 pairs of twins registered in Sweden , monozygotic and dizygotic, about health and personality aspects .

As in the Minnesota study, the twins in this Nordic research were also separated at birth and raised in different families. The control group used consisted of twins raised in the same family environment.

The results of this study gave strength to the idea that variation in cognitive aspects such as general intelligence shows that they are highly heritable , close to 70% in monozygotic twins.

As for the aspects more related to personality, such as the neuroticism dimension, there was an inheritance of about 50% among monozygotic, while in dizygotic twins there was a reduction of 20%.

3. Longitudinal study of Great Britain (2003)

Among its principal investigators is Robert Plomin. About 7,000 pairs of British twins were studied and IQ was measured . They measured the extent to which the family environment influenced the passage of time.

They were able to obtain data confirming the hypothesis that, as they grow up, twins (and people in general) are less influenced by the common environment, going from an influence of 75% to just 30% from adolescence to adolescence. adulthood

Advantages and limitations

As with all types of research, twin studies have led to certain advantages that have allowed us to have greater knowledge about genetics and the environment. But just as they have their benefits, they are also not exempt from limitations.

One of the advantages they have is the clearest: they allow to discern more clearly between the genetic factor and the environmental factor when studying a certain characteristic. In addition, the use of twins as an experimental sample improves the statistical capacity of genetic studies, reducing genetic and environmental variation (if the family is the same).

However, among the limitations they show is the fact that the population was not obtained randomly, as we are talking about couples of people, not individuals separately. Furthermore, it must be said that most studies of this type follow the same foundations as the first ones, close to centenarians.

On many occasions the results have been misunderstood or even misrepresented , not only by the media but also by the researchers themselves, favoring one of two nature versus debate positions. nourish’.

As for the characteristics of the sample, it must be said that people who participate in this type of study usually do it voluntarily. Assertiveness is a characteristic that, as most participants in this study show, it is difficult to discover to what extent this is due to a genetic component or a more environmental aspect, which may imply a certain bias.

We hope you have understood about the twin studies.