Twin research: what it is, how it works, and what it is for

An overview of how twin studies with the same genes work.

For quite some time now, attempts have been made to find out to what extent genetics and environment influence different aspects of human personality, behavior and cognitive abilities. However, genetics and environment are not two aspects that can be easily separated under laboratory conditions.

We cannot detach 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 certain 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 the hereditary factor to be controlled. and be able to differentiate it more clearly from environmental influences.

Let us take a closer look at what twin research or twin studies consist of.A type of natural study in which the ethical implications of separating a child from its family in laboratory conditions are not violated.

What are twin studies?

Twin research are tools whose sample consists of twins, either identical twins (monozygotic) or identical twins (monozygotic). 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 are the real influences of the environment and genetics on different characteristics of the human being, such as personality traits, cognitive abilities or incidence of psychiatric disorders. They can be considered natural experiments, since nature provides us with individuals in whom environmental variables can be isolated, since they have the same genes.

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

Galton, in an 1875 paper entitled The History of Twins (argues that nature, i.e., what we now call genetics, is the prevailing factor over environment, defending an innatist idea of both human behavior and personality. 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 to compare identical twins with fraternal twins, establishing the degree of heritability of certain grades according to the differences seen in them. according to the differences seen in them. The idea behind it was that whatever was seen in both identical twins had to be due to the genetic factor, especially if there was the unfortunate situation where these siblings had been raised separately.

• You may be interested in "Genetics and behavior: do genes decide how we act?"

What are they good 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 at early stages of development, generating not one but two fetuses from the same cell. Thus, this type of twins are genetically identical, and to say that they are the clone is, technically, true.

On the other side are dizygotic twins, also called in popular parlance 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 the same. The genetic relationship between these twins is the same as it would be with other siblings in the family, only they are born at the same time.only they are born at the same time. In fact, they may be of different sexes.

The usefulness of twin studies is especially related to monozygotic twins. This type of research tool makes it possible 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' debate more and more moderate. Today it is generally accepted that genetics and environment have similar weights, but in the past the positions were quite different.but in the past the positions were quite polarized. While Galton defended innatism and that nature was everything, Sigmund Freud's psychoanalysis said rather the opposite. Psychoanalysts defended the idea that disorders such as autism or schizophrenia were due to how children were raised.

In a nutshell, the methodology of twin studies consists in calculating the correlations of those traits under study, detecting concordances or differences. These are then compared between identical twins and fraternal twins. On this basis, if the heritability factor for a particular trait is high, monozygotic twins should be very similar with respect to that same trait. This genetic weight is especially important to measure in situations where twins have been reared separately.

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

These differences may be due to various factors, which would be within the non-shared or individual environment variable, such as different group of friends, different teachers, favorite parent... However, it is still argued that in monozygotic twins, having high genetic similarity, both tend to look for the same components for their individual environments, both tend to seek the same components for their individual environments..

Famous twin research

Below we explain three of the most famous twin studies. These studies investigated the heritability of several characteristics, both physical and characteristics, both physical and related to personality, mental disorders and cognitive abilities..

1. The Minnesota Study of Twins Reared Apart (1979 to present)

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

In this study it was assumed that the similarities shown by these twins were necessarily due to their genetic basis.. Of all the twins studied, one pair of twins was particularly striking and had a large number of coincidences:

• Their names: James Lewis and James Springer.
• Both married and divorced women named Linda.
• They remarried a Betty.
• Both received police training.
• They drank and smoked alike.
• They bit their nails.
• Their sons: James Alan Lewis and James Allan Springer.

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

The researchers found that about 70% of the variance in IQ in the sample had to be 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 household in such aspects as personality, mannerisms, social behavior, leisure and career interests.

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

Its principal investigator is Nancy Pedersen. Questionnaires were administered in which about 13,000 pairs of twins registered in Sweden, both monozygotic and dizygotic, were administered questionnaires asking 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 lent 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.The results of this study gave strength to the idea that the variance in cognitive aspects such as general intelligence is highly heritable, close to 70% in monozygotic twins.

As for 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 was down to 20%.

3. Longitudinal study from Great Britain (2003)

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

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

• You may be interested in, "What is Intellectual Quotient (IQ)?"

Advantages and limitations.

As with all types of research, twin studies have had certain advantages that have led to a greater understanding of genetics and environment. But, just as they have their benefits, they are not without their limitations.

Among the advantages they possess is the clearest: they make it possible to discern more clearly between the genetic factor and the environmental factor when studying a given trait. when studying a given trait. Furthermore, the use of twins as an experimental sample improves the statistical capacity of genetic studies, reducing both the genetic variance and the environmental variance (if the family is the same).

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. In addition, it should be said that most studies of this type follow the same fundamentals as the first ones that were carried out, which are close to a century old.

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

As for the characteristics of the sample, it should be noted that the people who participate in this type of study usually do so voluntarily. Assertiveness is a trait that, given that those who participate in this study show the most, it is difficult to ascertain to what extent it is due to a genetic component or to a more environmental aspect, which may imply a certain bias.

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.
• 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 (21 February 2012). "Minnesota Twin Family Study. Twin Research. 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