Twins under a magnifying glass - how the twin method is used in psychology

Types of studies involving twins - what's the difference?

The twin method is a priority when it is necessary to conduct research into behavioral characteristics based on genetics. The logic that guides scientists when choosing the twin method looks like this:

1. monozygotic twins have the same genetic makeup, dizygotic twins have a different genotype (as in children born from different pregnancies);
2. it is assumed that external factors (social situation in the family, parental attitudes, level of education, family traditions and others) have approximately the same influence on twins;
3. comparison of similar characteristics and differences within a twin pair allows us to establish the degree of influence of environmental and genetic factors on the severity of the trait being studied. If it is due to the characteristics of the genotype, then in monozygotic twins it should be much brighter than in dizygotic twins.

This logic marked the beginning of the use of the twin method in psychology . Today, other types of twin monitoring are used.

Matching twins with non-twins

Also an auxiliary method to assess the significance of the difference between twins and non-twins. If the difference between twins and other people is not significant, then twins and other people belong to the same general sample and, therefore, the results of twin studies can be generalized to the entire population.

Thus, some developmental lag between members of twin pairs and singletons was noted. This difference is especially noticeable at an early age. But a comparison of the results of a study of members of twin pairs whose partner died in early childhood and singletons did not reveal a significant difference in the level of development.

That is, the peculiarities of the development of twins are determined not so much by the difficulties of embryonic development, but by the peculiarities of raising twins as a pair (family difficulties when giving birth to twins, isolation of twins in a pair, etc.). Thus, twins are somewhat different from the rest of the population, but with age this difference is noticeably smoothed out and twins, for the most part, become comparable to the rest of the population.

Method of separated twins : Due to the peculiarities of the development of DZ and MZ twin pairs, the classical twin method and its variations are considered to be “non-rigid” experiments: in them it is impossible to unambiguously separate the influence of genetic and environmental factors, since for a number of reasons the conditions for the development of twins for a number of reasons reasons turn out to be incomparable.

Partially separated twin method : This method consists of comparing the intrapair similarity of MZ and DZ twins living apart for some time.

In these studies, it is also possible to determine to what extent the postulate about the equality of the environments of MZ and DZ twins is true. Thus, if MZ twins living separately become less and less similar to each other in terms of some psychological characteristic, and DZ twins living apart do not differ in intra-pair similarity from DZ twins living together, then we can conclude that the environmental conditions of MZ and DZ are unequal, and conclusions about the heritability of the characteristic being studied overestimate the heritability of this characteristic.

How the twin method was used by Russian scientists

The pioneer in the application of the twin method was S.A. Sukhanov in 1900. His goal was to find out how genetics and the characteristics of the environment in which the members of the couple live influence the occurrence of psychosis.

Scientists from the Medical Biological Institute continued his work in the 30s of the last century. They actively used the twin method to study the inheritance of a tendency to somatic pathologies (respiratory, endocrine and digestive systems).

Research by the institute's scientists has made it possible to learn about the mechanisms that contribute to the emergence of psychological characteristics in childhood and the influence of hereditary factors on the ability to learn. Scientists paid special attention to the study:

• short-term and long-term memory;
• attention;
• intellectual abilities;
• motivational sphere;
• strategies for achieving the desired results.

The main attention of researchers was aimed at studying how active learning affects the formation of interest in knowledge, the development of intelligence and the formation of a harmonious personality. Using the control twin technique, the scientists tested learning performance on the other member of the pair. Upon completion of the research, scientists came to the conclusion that frequent repetition of simple or more complex manual actions with objects in preschool children does not increase the rate of knowledge acquisition. Manipulation cannot be considered the main stimulator of child development.

Genes and environment: how twins help in the study of behavior

1. What is the essence of the twin method?

Twin method or twin method

- a key method used in psychogenetics (the science of heredity and environmental influences on behavior).

The method consists of comparing identical (monozygotic - MZ) and fraternal (dizygotic - DZ) twins. Monozygotic twins are actually 100% genetic copies of each other, and dizygotic twins are on average 50% genetically similar, like normal siblings. Moreover, both twins share a common environment, for example, they develop in the same womb, are raised in the same family, and often study in the same school or even in the same class.

This means that differences in physical and psychological characteristics and behavior between monozygotic twins are due to environmental influences, and differences in dizygotic twins are due to both environmental and genetic influences. The twin method is aimed at identifying environmental and genetic influences on various traits.

Modern twin studies indicate that almost all traits are influenced in part by genetic factors, with some traits showing stronger genetic influences (such as height) and others showing less strong genetic influences (such as personality traits). Most traits are complex - their heritability is determined by a large number of genes, for example, in the case of autism.

Illustration by Georgy Sapego

2. Who discovered the twin method?

The idea of ​​the twin method was formulated by F. Galton back in 1865. And in 1876 his article “The History of Twins as a Criterion for the Relative Strength of Nature and Nurture” appeared. The article discusses the biology of multiple births, the twin method of genetics, and the data obtained with its help about the role of “nature” and “nurture” in the formation of individual characteristics of people.

In Russia, the first systematic psychogenetic studies using twins began half a century later - in 1929 at the Medical Genetics Institute. The research group led by S.G. Levita created the largest (at that time) twin registry in the world, including more than 1,000 pairs of twins. The scientists also set up a “twin” kindergarten to conduct research. However, this work was stopped and banned in the late 1940s as a result of the “Lysenkoism”. Subsequently, research in the field of behavioral genetics in the USSR ceased for 25 years.

3. What discoveries have been made using the twin method?

The twin method is used in healthcare to study the processes of heritability of various diseases and predispositions, and in the humanities. There are a number of large-scale longitudinal twin studies in the world today.

For example, a number of psychogenetic studies analyze individual differences in learning abilities and school performance in individual subjects. Thus, for reading ability, heritability estimates of about 40-60% have been obtained, and estimates of the influence of the general environment vary from 25 to 45%. A similar contribution of genetic and environmental factors to individual differences was also obtained for abilities in mathematics. Other twin studies have examined individual differences in number sense (the ability to correctly estimate the number of objects at first sight without counting them). Individual differences in number sense are detected early in life, persist into later development, and are associated with general intelligence. Research has shown that the genetic contribution to number sense is only 32%, with individual differences largely due to unshared environmental influences.

Another interesting study was conducted under the leadership of Julia Covas. It was attended by 13 thousand twins aged 9 to 16 years from Russia, Great Britain, the USA, Germany, Japan and Canada. Scientists tried to answer the question of what are the reasons for schoolchildren’s motivation to learn, why they like or do not like to study, and how interesting this or that subject of the school curriculum is to them. The study results were published in the journal Personality and Individual Differences. It turned out that even monozygotic twins who grow up in the same family and study in the same class can differ greatly in their motivation. According to the head of the study, this suggests that there are a lot of environmental influences that do not work at the family or class level, but are individually specific. Genetic factors also contribute to individual differences in motivation.

4. What types of twin method are used today?

There are several main types of twin method: classical (comparing the similarity of monozygotic and dizygotic twins), separated twin method, study of families (for example, children) of MZ twins, control twin method, twin pair method.

Separated twins method

consists of assessing the intrapair similarity of the trait under study in twins who were separated in childhood and, therefore, raised in different environments. The most valuable are pairs of MZ twins (MZt) separated in the first months or years of life, since this means that two genetically identical people grow up in different conditions. In this case, their similarity cannot be explained by the similarity of the postnatal environment; therefore, the resulting correlations largely reflect the heritability of the trait.

A criticism of this version of the twin method is the difficulty of assessing the actual differences in the environments in which MZ twins were raised.

MZ twin family method

, or the monozygotic half-sibling method, involves matching children in families where the mothers or fathers are monozygotic twins. These children have a unique genetic status: aunt (or uncle), from a genetic point of view, is the same as mother (or father).

The main task for which this method provides valuable material is to identify the so-called maternal effect. This term refers to the predominant influence of the mother on the phenotype of the offspring, which arises not only due to the transmission of genes, but also due to psychological, postnatal, as well as biological and intrauterine factors.

The presence of a maternal effect in a psychological trait should manifest itself in greater similarity between children with MZ twin mothers compared to children with MZ twin fathers. Also of interest are the effects of cytoplasmic heredity, which can be detected (in human psychological characteristics) so far only using the method of families of MZ twins. The fact is that, in addition to the genes contained in the chromosomes of the cell nucleus, some structures located in the cytoplasm are carriers of hereditary information.

Control twin method

(mutual control of twins, control by partner, twin-witness) is based on the fact that MZ twins are ideal controls for each other. If the group of MZ twins is divided into two samples so that each includes one twin from each pair, then the researcher will receive a unique situation - two samples, equalized both in genetic constitution and in basic environmental (family-wide) parameters. Then you can, for example, train one group and not the other, or train them at different ages, as was done in the work of A. Gesell, who first used the method by studying the development of one female pair of MZ twins from the first months of life to 14 years. They received material indicating that the effectiveness of learning (motor and speech) depends on age.

Twin pair method

. Back in the 30s, works appeared that talked about a special psychological situation in pairs of twins, the “couple effect”, or the “twinification effect”. The main characteristics of this special psychological situation are two opposing tendencies: towards identification with one’s co-twin and, on the contrary, towards individualization of each twin. The first leads to the feeling of being first a member of a couple and only then - a separate person; the second, in extreme cases, can lead to pronounced conflict relations between twins, a protest against the “second one is the same.” With age, the first tendency usually weakens, giving way to the second, and in male twins this process occurs more easily than in females.

5. I'm a twin - how to become a research participant?

The Laboratory of Cognitive Research and Psychogenetics at Tomsk State University is one of the key research centers in Russia where the twin method is actively used. In 2011, on the basis of the laboratory, with the participation of the Russian-British Laboratory of Psychogenetics of the Psychological Institute of the Russian Academy of Education, the first twin registry in Russia was created - the Russian school twin registry “Progress”.

The Gemini Register is an important resource and tool for studying how individual differences influence learning. With its help, world-class interdisciplinary research is brought to life, molecular genetic cross-cultural research is conducted to understand the mechanisms of cognitive development.

Parents of twins and teachers of secondary schools are invited to participate in this project. To register school-age twins, you must fill out the registration form on the website.

And in 2015, an international scientific research project was launched aimed at studying the nature of individual differences in cognitive processes. The purpose of the study in the long term is to analyze the reasons for the success and failure of people in mastering mathematics and other cognitive functions. The goal is also to develop new, more “mathematically friendly” education programs from school to university. The project is carried out in collaboration with the InLab, Goldsmiths, University of London; laboratory of developmental psychogenetics and Russian-British laboratory of psychogenetics of the Psychological Institute of the Russian Academy of Education; and the Ohio University Innovation Center for Learning Disabilities Research. As part of the project, research into the neurophysiological mechanisms of individual differences in cognitive processes is carried out using the twin method.

Illustration by Georgy Sapego

If you are twins between the ages of 17 and 30 and want to take part in this project, then you need to fill out the online registration form, which you can find on the official project page here.

Also, all information about the activities of the Laboratory of Cognitive Research and Psychogenetics, as a structural unit of the International Center for Research in Human Development, can be obtained on the website.

What does the twin method allow you to study?

Comparison of similarities in a pair of twins born from single- and double-fetal pregnancies allows us to draw a conclusion about the influence of the genotype on the manifestation of specific traits. Thus, in psychology, the twin method makes it possible to determine the degree of influence of biological or environmental factors on the following characteristics:

• intelligence;
• memory;
• Creative skills;
• behavioral characteristics;
• tendency to addiction;
• behavioral stereotypes;
• psychical deviations.

Scientists studying the psychological characteristics of twins in Britain and the USA have the concept of the “Big Five”. It refers to 5 priority characteristics that are studied especially carefully. Among them are:

• Neuroticism is a characterological feature, a predisposition to fixation on negative emotions - anger, anxiety, irritability, emotional instability, depression. People with high levels of neuroticism have difficulty coping with stressful situations, interpreting everyday events as dangerous, insurmountable and hopeless. Neuroticism is one of the most well-studied and experimentally supported personality traits. Many studies confirm the large role of heredity and the appearance of the first symptoms in childhood.
• Extraversion is a characteristic of a personal orientation towards the external world, polar to introversion (direction towards the inner world). The genetic basis of extraversion is supported by most twin studies.
• Openness to new experiences is a personality characteristic that is opposite to resistance and isolation. Characterized by the ability to perceive new ideas, feel comfortable in a new environment, when communicating with new people. Most researchers consider openness to be a genetically determined characteristic.
• Agreeableness - some scientists note a high correlation between the formation of benevolence as a character trait and parental love, discipline, while others indicate that benevolence is an indicator that is inherited in 30% of cases.
• Mindfulness is the ability to focus on what is happening in the present moment, to monitor current experiences, without being distracted by thoughts about the past and future. Twin studies indicate heritability of mindfulness in unseparated couples.

It is known that the high similarity of identical twins allows us to draw a conclusion about the leading role of genetics in the emergence of certain characteristics. When similarity is high in identical twins, but low in fraternal twins, scientists say that the genetic factor plays a certain, but not leading role.

In the case when the similarity indicators in twin pairs of different types are relatively the same, it is concluded that genetics does not play a major role in the formation of specific characteristics; environmental factors come to the fore.

Determining the relationship between hereditary and environmental factors on the occurrence of pathological tendencies makes it possible to predict the occurrence of serious problems and carry out preventive measures in a timely manner.

Identification of monozygotic twins

To identify monozygotic twins, the criteria for diagnosing twin zygosity are used - phenotypic characteristics determined only by the genotype.

These signs include the following:

• gender, • blood groups according to different systems (ABO, Rh, MN, etc.), • eye and skin color, • hair color and shape, • shape of nose, lips, mouth, • shape and size of head, ears, fingers and hands, • structural features of the teeth, the color of their enamel, • location of freckles, skin vessels, • skin patterns on the fingers and palms. If these characteristics are the same in two twins, then we can confidently say that they are monozygotic twins.

It is better to identify twins several years after birth, since in childhood some signs are not clearly expressed enough.

In obstetric practice, the type of twins is determined by the number (one, two) of amniotic membranes and placentas. In addition, a reliable criterion for the monozygosity of twins is the engraftment of pieces of skin transplanted from one twin to the other.

Twin method: options for monitoring twins

Researchers use different variations of twin studies. Experts make the choice of a specific method based on the features they plan to study.

Methods for observing twins differ in the degree of reliability and conditions of implementation, therefore the degree of accuracy of the results also varies. But even though the results of studies cannot always be compared, the use of the twin method remains a priority when it comes to studying the influence of environmental factors.

Classical

The use of the classical technique involves comparing the results of observation of identical and fraternal twins. The classical technique is based on a comparison of intra-pair differences in behavior that are monitored over many years (longitudinal study).

This type of experiment makes it possible to identify significant environmental and hereditary factors in the development of the subjects being studied.

Control twin

This technique is used to identify the leading environmental factors that influence the formation of a specific psychological trait. It is advisable to use the technique only when the absolute genetic similarity of two monozygotic twins is proven. The couple is separated, one of the participants is placed in the experimental group, and the other in the control group. At the end of the experiment, differences (only significant ones) obtained under the influence of participants in the experimental group are identified and compared with the same parameters obtained in the control group.

If the same parameters are found in both groups, the result of the study is reliable.

Separation of twins

This technique is the most cruel. For the study, twins (unizygotic and fraternal) are separated shortly after birth. They often have no idea that they have a brother or sister. Children have genetic similarities, but the conditions in which they develop can be diametrically different.

Cases of twins being separated for research are few and far between, and scientists lack the ability to control for spillover effects. Because of this, it is believed that this method is not reliable enough. Today it is practically not used due to ethical considerations.

Twin families

This technique is a combination of family and twin studies. It is used when it is necessary to study how hereditary factors influence the occurrence of mental disorders.

The objects of twin family research are children and grandchildren of adult twins. The method also makes it possible to determine the degree of influence of maternal and paternal genetics on the psychological development of children who are not twins.

Single

The technique involves observing one surviving twin from a pair. This type of research is used to determine the role of intrauterine development in later life and the characteristics of a person’s personal development.

The study is conducted through observation and makes it possible to assess the influence of other factors. Typically, the single-twin method is used in conjunction with a classical type study.

Methods in human genetics. Biochemical method. Dermatoglyphics

In contrast to the cytogenetic method, which makes it possible to study the structure of chromosomes and karyotype normally and to diagnose hereditary diseases associated with changes in their number and disruption of organization, hereditary diseases caused by gene mutations, as well as polymorphism in normal primary gene products, are studied using biochemical methods.

These methods were first used to diagnose genetic diseases at the beginning of the 20th century. Over the past 30 years, they have been widely used in the search for new forms of mutant alleles. With their help, more than 1000 congenital metabolic diseases have been described. For many of them, a defect in the primary gene product was identified. The most common among such diseases are diseases associated with defects in enzymes, structural, transport or other proteins.

Defects in structural and circulating proteins are identified by studying their structure. So, in the 60s. XX century An analysis of the 3-globin chain of hemoglobin, consisting of 146 amino acid residues, was completed. A wide variety of hemoglobins in humans was established, associated with changes in the structure of its peptide chains, which is often the cause of the development of diseases (see § 4.1).

Enzyme defects are determined by determining the content of metabolic products in the blood and urine that are the result of the functioning of this protein. A deficiency of the final product, accompanied by the accumulation of intermediate and by-products of impaired metabolism, indicates an enzyme defect or deficiency in the body (see § 4.1).

Biochemical diagnosis of hereditary metabolic disorders is carried out in two stages. At the first stage, presumptive cases of diseases are selected, at the second stage, the diagnosis of the disease is clarified using more accurate and complex methods. The use of biochemical studies to diagnose diseases in the prenatal period or immediately after birth makes it possible to timely identify pathology and begin specific medical measures, as, for example, in the case of phenylketonuria.

To determine the content of intermediate, by-products and final metabolic products in the blood, urine or amniotic fluid, in addition to qualitative reactions with specific reagents for certain substances, chromatographic methods for studying amino acids and other compounds are used.

Twin method: main problems

Studies involving twins are often fraught with problems related to the influence of external factors, but the main difficulty is the selection of mono- or dizygotic participants. The problem is mainly due to the insufficient regulation of legal issues regarding access to twin registries. In the USA and Western European countries, when children are born in couples, they are included in special lists, and parents are immediately asked for permission to participate in research.

In the countries of the former Union there is no such practice. Scientists don't have access to official data on twins, so inviting them to participate in a study can be very difficult.

Monozygotic

Monozygotic or identical twins are those who are born as a result of the fertilization of one egg and one sperm and the subsequent division of the zygote into two completely identical parts.

Identical twins are natural clones. Their gender, genetic make-up, and blood type are the same. The appearance of monozygotic twins is also almost identical - the differences are very difficult to notice. Intrauterine development occurs within the same placenta.

Approximately a quarter of monozygotic twins have such a feature as mirroring. This can be seen by the fact that their moles are located on opposite parts of the body. Examination often reveals the opposite arrangement of internal organs.

Siamese twins born conjoined are always identical.

Dizygotic

Twins, called dizygotic (twins), are born as a result of fertilization of several eggs by different sperm. Intrauterine development occurs within different placentas.

The genetic makeup of dizygotic twins is not identical. It, just like in children born from different pregnancies, is only 50% the same. Those born in a dizygotic couple are not alike in appearance and may be of different sexes.

In this case, twins undergo simultaneous development before birth and after birth. Therefore, they differ from children born in the same family from different pregnancies.

Group distribution

In experimental psychology, methods are used to distribute pairs of twins depending on which parameter is planned to be studied and to what extent.

For example, to study the level of verbal intelligence, twins from different groups can be used:

• those whose partners died in utero;
• whose partners died in childhood;
• those who were raised together.

The formation of such groups makes it possible to confirm or refute the hypothesis that the situation in which twins develop is reflected in their speech development.

When forming groups, scientists always focus on the fact that the more precisely the goal of the experiment is set and the participants are correctly selected, the more accurate data will be obtained.

Application of the method

The introduction of the similarity method makes it possible to enter the nature of zygosity with a fairly high degree of reliability. In practice, egg identification using the similarity method is performed when the twins are same-sex. Unlike ordinary pairs, as a rule, each time the result is ambiguous, and it must be correctly established.

The exceptions are those exceptional cases when sex chromosome analysis is done from identical pairs . If the flaw is not available, there are no exceptions to this rule. If twins are same-sex, then they have every chance of being unilateral or bilateral. Identity on several symptoms selected by the researcher speaks in favor of their identity. The presence of differences in character allows us to conclude that in fact the twins are considered bi-identical.

The likelihood that identical ones will be the same to the same extent as similar ones is very small. An absolute match between identical twins on the 8 symptoms studied is quite likely in about 1 in 480,000. If the number of matching symptoms is greater, then, of course, the probability of perfect homogeneity for identical twins is even less.

Determination of blood groups and serum proteins guarantees an even higher level of reliability when comparing when the material is used.

Advantages of the twin method

Research involving twin pairs makes it possible to obtain answers to questions concerning various aspects of the formation of the psyche, intelligence, and interpersonal interaction.

The twin method is used in various branches of psychological science - ontopsychology, criminal and medical psychology, and others. Let us also apply the method in sciences close to psychology:

• medicine;
• genetics;
• pedagogy;
• physiology;
• biology.

The twin method makes it possible to identify the main differences between members of a couple, as well as to determine the main factors that contribute to their personal development.

Twin Birth Rate

The overall incidence of twin births averages 1.1 – 1.2% of all births; of these, about 1/3 are monozygotic twins, and 2/3 are dizygotic twins.

The incidence of monozygotic twins is similar across populations, but the incidence of dizygotic twins varies significantly across populations.

For example, in the United States, dizygotic twins are born more often among blacks than among whites. In Europe, dizygotic twins occur at a rate of 8 per 1,000 births.

The lowest frequency of birth of dizygotic twins is inherent in Mongoloid populations, where it is 2-2.5 per 1,000 births

The likelihood of having dizygotic twins increases with the age of the mother and the birth order of the children.

Due to the slightly increased mortality rate among twins compared to that of singletons, the proportion of twins in the population is only 0.9%.

Diagnosis of zygosity

When selecting research participants, a mandatory diagnosis of zygosity is carried out. It requires knowledge of the number of placentas; it is necessary to compare twins on the basis of external similarity and internal markers - blood type, genetic makeup, which is determined by DNA analysis.

When children are born in separate placentas that connect them to their mother, we are talking about a dizygotic relationship. During intrauterine development, fetuses are wrapped in two membranes - amnion (internal) and chorion (external).

The situation with monozygotic twins may be different. They develop separate placentas and chorions or a common chorion. If both fetuses develop in a common chorion, then they have complete genetic similarity.

Diagnosis of zygosity determines the degree of differences. If genetic tests show differences between members of a couple, then we are talking about zygosity.

Homozygosity is a probabilistic diagnosis. It is determined by the results of several tests - the more matches they reveal, the more likely the fact of homozygosity is.

Irina Sherbul

The concept of monozygotic and dizygotic twins

Twins are children born at the same time. Monozygotic (identical) twins develop from one zygote - a fertilized egg, and only one sperm takes part in fertilization of the egg.

Usually one embryo develops from a zygote. But sometimes, after the first mitotic division, two blastomeres separate from each other and begin independent development: from each blastomere an embryo separate from the other is formed, as a result of which the mother gives birth to two children identical in gender, genotype and phenotype.

The very slight differences they have are due to small hereditary and non-hereditary changes that arise in their somatic cells during individual development.

Diagram explaining the development of monozygotic boisenes.

1 – sperm 2 – egg 3 – zygote 4 – blastomere.

Dizygotic (fraternal) twins develop from different zygotes when two different eggs simultaneously leave a woman’s ovary and are each fertilized by their own sperm.

Dizygotic twins, due to the combinative variability that arises during gametogenesis in the parents, differ from each other in the sets of specific chromosomes and gene alleles and, therefore, in their genotypes and phenotypes.

Dizygotic twins born at the same time are similar to each other and differ from each other to the same extent that children of the same parents born in different years are similar to each other in many ways and at the same time differ from each other.

The only difference between them is that dizygotic twins are born at the same time, and not with an interval of several years, like ordinary siblings.

Diagram explaining the development of dizygotic males. 1 – sperm 2 – egg 3 – zygote 4 – blastomere.

Background

F. Galton was the first to establish this method in 1875. In his article on the history of twins as a criterion of relativity in cultural and genetic approaches, he considered them as a research tool with the aim of finding individual differences in the context of the influence of biological and environmental factors on the formation of personality. However, before him, some thoughts about this research method were expressed. Following the publication of his paper, academics around the world criticized his position, hinting at “deep logical flaws” in his theory. External (biological) and internal (psychological) similarities are difficult to separate according to the nature of their formation and to state as a result of which this or that characteristic is formed. The role of heredity and social environment in the formation of personality is the problematic aspect for which the twin research method was invented.

Already in the mid-20s of the twentieth century, psychologists began to develop and use improved mechanisms and methods for studying twins. The main new method is to compare the properties of identical and dizygotic pairs. In comparison, if identical twins are more similar on a personality trait under study than fraternal twins, then the trait is likely to be more heritable. Such studies are being used more and more often, and the field of problems is quite large.

Sources of Confounding Variable Effects

A common source of error when applying the method is the lack of environmental corrections . Through research, it was revealed that parents do not treat each member of the couple equally, despite obvious external differences.

Monozygotic twins are identical in appearance and therefore evoke similar emotions and, therefore, attitudes in their parents.

And in the case of a dizygotic couple, parents raise children differently, as they notice and react to external differences .

This factor may distort the results of the study.

Inaccuracies may also arise due to erroneous determination of the level of genetic similarity .

In this case, the researcher will make an incorrect conclusion based on the results, since he will initially have false information.

Read about the concept and meaning of empirical validity in psychology and psychodiagnostics here.

Whose names are the development associated with?

The founder of the twin method is the scientist Francis Galton , who proposed this concept in 1875, and in 1876 published his ideas in a work on the forces of nature and nurture and their relativity using the example of twins.

However, his approach to the issue had serious logical inconsistencies .

After Galton's work was published, representatives of the world of science criticized Francis's assumptions, hinting at the huge number of logical flaws found in the article.

E. Thorndike also studied twins , who took fifteen pairs of twins and their single sisters/brothers as a basis, and then conducted a series of tests and compared the results. As a result, the scientist revealed the role of heredity in the individual mental characteristics of an individual.

The closest approach to the actual twin method was S. Merriman, who, using the Stanford-Binet test, divided teenage twins into two groups (duplicate and fraternal).

The methodological basis was formed by Siemens in 1924. He also derived sampling rules for further implementation of the method.

Features of the event

The twin research method is carried out in three stages :

1. Sampling.
2. Diagnosis of zygosity.
3. Evaluation of the results obtained by comparison.

During the sampling process, a specialist must examine each experimental subject within each pair.

If it is not possible to examine one of the twins, the sample is considered invalid.

It is also necessary to take into account the percentage of multiple births for each race and people in order to eliminate inaccuracy in the results.

The age and gender of twins in the same sample is also an important criterion, since differences in these characteristics can negatively affect the validity of the study results.

The type of zygosity can be determined by studying the number of fetal membranes or by polysymptomatic similarity.

The latter method was introduced by Verschuer and Siemens. It is based on the principle of similarity of morphological features .

There is another method (Smith and Penrose), based on the principle of analyzing zygosity by blood groups of twins.