Research with twins: what they are, how they work and what they are for

For quite some time now there has been an attempt to find out to what extent genetics and environment influence different aspects of the personality, behavior and cognitive abilities of the being human. However, genetics and environment are not two aspects easily separable under laboratory conditions.

We cannot separate a person from his family with the intention of controlling all the stimuli he receives, nor Nor can we genetically modify it to see to what extent one or more genes are behind a certain feature.

Fortunately for scientists, twins exist, especially those who, for various reasons, have been separated from each other. With the same genetics, Identical twins allow the hereditary factor to be controlled and to be able to differentiate it more clearly from the influences of the environment.

Let's see in more detail what research or studies with twins consist of., a type of natural study in which the ethical implications of separating a child from her family under laboratory conditions are not violated.

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What are twin studies?

Research with twins is a tool whose sample is made up of twins, whether they are identical twins (monozygotic) or fraternal twins (dizygotic).

Both in the last century and in the current one, several of these studies have been carried out, with the intention of finding out what the real influences of the environment and genetics on different characteristics that human beings manifest, such as personality traits, cognitive abilities or incidence of disorders psychiatric. They can be considered natural experiments, given that nature provides us with individuals in whom environmental variables can be isolated, by having the same genes.

The origins of this type of studies are in the classic debate about what is more important in terms of the development of people, whether the environment or the environment, better known in English as the 'nature vs. nurture'. The first to propose using twins to study the heritability of traits, both physical and psychological sciences, comes from Sir Francis Galton, best known for being the cousin of Charles Darwin and a pioneer in the ideas eugenics.

Galton, in an 1875 document entitled The History of Twins (The Story of Twins), defends that nature, that is, what we call genetics today, is the factor that prevails over the environment, defending an innate idea of ​​both behavior and personality human. With the passage of time, in the 1920s, the methods proposed by Galton were perfected.

In these first studies the intention was to compare identical twins with fraternal twins, establishing the degree of heritability of certain grades depending on the differences seen in them. The idea behind it was that whatever was seen in both identical twins had to be due to the genetics, especially given the unfortunate situation in which these brothers had been raised by separate.

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What are they 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 having split in early stages of development, generating not one but two fetuses from the same cell. Thus, these types of twins are genetically identical, and saying that they are the clone is, technically, a truth.

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

The usefulness of twin studies is especially related to monozygotic twins. This type of research tool allows us to control a factor that would be impossible to control in other types of people: genetics. That is, it is possible to compare two people who have the same 'nature', as Galton would say, in order 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 vs. nurture’ becomes increasingly moderate. Nowadays it is generally accepted that genetics and environment have similar weights, but in the past the positions were quite polarized. While Galton defended nativism and that nature was everything, psychoanalysis led by Sigmund Freud said quite the opposite. Psychoanalysts defended the idea that disorders such as autism or schizophrenia were due to how children were raised.

In summary, the methodology of twin studies consists of calculate the correlations of those traits that are the object of study, detecting concordances or differences. Subsequently, these are compared between identical twins with those who are fraternal. Based on this, if the heritability of a specific trait is high, monozygotic twins would have to be very similar with respect to that same trait. This genetic weight is especially important to measure in situations where twins have been raised separately.

An idea that has been defended with respect to this type of research is the fact that it is possible find out how the family environment, also called shared, loses weight as the person ages growing. This phenomenon is especially striking in families where there are twins, whether fraternal or identical, since as they grow it is easier to see to what extent they are different of the other.

These differences may be due to various factors, which would be within the non-shared or individual environment variable, such as different groups. of friends, different teachers, favorite father... However, it is still defended that in monozygotic twins, having 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 carried out with twins. They investigated the heritability of several characteristics, both physical and related to personality, mental disorders and cognitive abilities.

1. The Minnesota Study of Twins rearranged apart (1979 to present)

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

In this studio It was assumed that the similarities that these twins manifested were necessarily due to their genetic basis.. Among all the twins studied, one pair that had a large number of coincidences drew special attention:

• Their names: James Lewis and James Springer.
• They both married and divorced women named Linda.
• They remarried a Betty.
• Both received police training.
• They drank and smoked in a similar way.
• They bit their nails.
• His children: James Alan Lewis and James Allan Springer.

And all these details are not the only ones. Coincidences of this type are rare, but they can certainly give strength to those who think that everything is nature before environment.

The researchers found that About 70% of the variance in the sample's IQ must have been due to a strong genetic component..

The study found that twins separated at birth and raised separately were as similar as twins. raised in the same home, in aspects such as personality, gestures, social behavior, leisure and interests professionals.

2. The Swedish Adoption/Twin Study of Aging (SATSA) (1980s and 1990s)

Its principal investigator is Nancy Pedersen. Questionnaires were administered where About 13,000 pairs of twins registered in Sweden were asked about health and personality aspects, both monozygotic and dizygotic.

As with the Minnesota study, the twins in this Nordic research had also been 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 The variance in cognitive aspects such as general intelligence shows that they are highly heritable, close to 70% in monozygotic twins.

Regarding aspects more related to personality, such as the neuroticism dimension, among monozygotic twins there was a heritability of about 50%, while in dizygotic twins this dropped to 20%.

3. Great Britain Longitudinal Study (2003)

Among its main researchers can be found Robert Plomin. Nearly 7,000 pairs of British twins were studied, and their IQ was measured.. They measured the extent to which the family environment had an influence over time.

They were able to obtain data that confirmed the hypothesis that as we grow older, twins (and people in general) become being less influenced by the common environment, going from an influence of 75% to only 30% from adolescence to adulthood adult.

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Advantages and limitations

As with all types of research, studies with twins have provided certain advantages that have allowed us to have greater knowledge about genetics and the environment. But, just as they have their benefits, they are not without limitations.

Among the advantages they have is the clearest: allow us to discern more clearly between the genetic factor and the environmental factor when studying a certain trait. Furthermore, the use of twins as an experimental sample allows us to improve the statistical capacity of the genetic studies, reducing both genetic and environmental variance (if the family is the itself).

However, among the limitations they show is the fact that the population has not been obtained randomly, since we are talking about pairs of people, not separate individuals. Furthermore, it should be said that the majority of studies of this type follow the same foundations as the first ones that were carried out, close to a century ago.

On many occasions the results have been misinterpreted or even distorted., not only by the media, but also by the researchers themselves, who favor one of the two positions of the 'nature vs. nurture’.

Regarding the characteristics of the sample, it should be said that people who participate in this type of study usually do so voluntarily. Assertiveness is a trait that, given that those participating in this study mostly display, is difficult to find out to what extent it is due to a genetic component or a more environmental aspect, which may imply some bias.

Bibliographic references:

• Andrés Pueyo, A. (1997). Heredity and the environment in the determination of individual differences. In Manual of differential psychology (ch. 11). Madrid: McGraw-Hill.
• Bouchard, Thomas J. Jr.; Lykken, David T.; McGue, Matthew; Segal, Nancy L.; Tellegen, Auke (1990). Sources of human psychological differences: the Minnesota study of twins reared apart. Science. 250 (4978): 223–8.
• Iacono, William G.; McGue, Matt (February 21, 2012). "Minnesota Twin Family Study." TwinResearch. 5 (5): 482–487
• Sahu, M., & Prasuna, J. g. (2016). Twin Studies: A Unique Epidemiological Tool. Indian journal of community medicine: official publication of Indian Association of Preventive & Social Medicine, 41(3), 177–182. doi: 10.4103/0970-0218.183593