Twins and the twin method

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.

World of Psychology

V.V. SEMENOV

Research Institute of General and Pedagogical Psychology of the USSR Academy of Pedagogical Sciences, Moscow

In studies of genetic and environmental influences on the development of a particular trait in humans, the twin method is usually used. It is known that one of the postulates of the twin method is the assumption of equality of environment for members of a monozygotic (MZ) pair, having an identical genotype, and a dizygotic (DZ) pair, whose genotype is not the same and differs in the same way as that of ordinary brothers and sisters in one family. It is assumed that in the absence of sharp differences in the environment of twin pairs, all variations in the development of MZ twins are determined by heredity.

In recent years, a large number of works have appeared in which the assumption of equality of environment for MZ and DZ pairs is questioned. There is evidence that the environment of members of an MZ couple is more similar compared to the environment of DZ twins. Nevertheless, it must be recognized that two independent lines of research are being conducted in psychogenetics: some analyze individual variables of both the environmental and intrapair environment that have an effect on the similarities and differences in twin pairs, while others do not take into account the uniqueness of the environment of MZ and DZ twins genetic determinants of personality variables are studied using various tests, questionnaires, etc. and in the case of greater similarity of a trait in MZ, a conclusion is made about its heritability. Analysis of the unique development of MZ twins in comparison with DZ and the ensuing limitations in the use of the twin method in psychology is the subject of this article.

Already the features of the prenatal environment of MZ and DZ twins are considered as factors that can lead to noticeable distortions in the assessment of the relative role of heredity and environment in the determination of psychological traits [3]. A number of studies have shown that the differences that exist in MZ twins in the prenatal period may be the basis for the development of individual characteristics of the members of the MZ pair. While not genetic, these differences persist throughout the twins' lives [10], [22], [34], [35]. However, some authors note [5], [7] that prenatal differences normally do not introduce noticeable distortions into the assessment of the heritability of individual psychological parameters. A much more serious criticism of the postulate of equality of environment for MZ and DZ twins concerns the dissimilarity of their postnatal development. Next, we will focus on data on the characteristics of the postnatal environment for MZ and DZ twins.

In unique experimental works by Newman, Freeman, Holzinger [28], Shields and Juel-Nilseia [26], MZ twins separated from early childhood were studied. The results of the study of separated twins were then compared with the data of MZ and DZ twins raised together [28], [33]. The advantage of such studies is the study of genetically identical individuals who grew up in different environmental conditions and have no experience of twin relationships. The main working hypothesis in this case is the following: the similarity of the partners of an MZ couple separated in early childhood is due to an identical genotype, and the differences between them are due to unequal conditions of their development. It turned out that on some indicators (in particular, extraversion), separated MZ twins received more similar scores than non-separated MZ twins [33]. At the same time, for many other characteristics, MZ partners who grew up separately received lower intrapair similarity scores than MZ who grew up together, but higher than DZ twins. Based on these data, it can be assumed that the similarities and differences of some psychological signs found in conventional twin studies may be due to the peculiarities of the coexistence of twins, the specific relationships that develop between members of a twin pair, the attitude of others towards them, primarily parents, and others .

The special position of the twins of the MZ couple in the family, the uniqueness of their intra-pair relationships is determined by various reasons. Gifford et al. [22] consider the individual development of twins as a result of the interaction of congenital constitutional differences, parental attitudes towards individual differences in children (parents can either enhance or reduce their significance) and relationships between the twins themselves, which tend to either increase the twins’ similarities or diverge in their development.

Von Bracken [13] notes in his review of twin studies of psychological functions that the developmental characteristics of twins largely depend on: the attitude of parents towards twins, which, as a rule, differs from the upbringing and attitude towards single-born children; from the reaction of surrounding people to the partners’ MH, emphasizing the similarities and perceiving them as a single whole; and from the interconnected, intimate coexistence of MZ twins.

Zazzo [37] believes that when studying the psychology of twins, three factors are usually taken into account: 1) the similarity inherent in MZ twins due to the identical genotype; 2) the similarity that the environment, understood in the broad sense of the word, gives them; 3) dissimilarity generated by the distribution of roles in a twin pair. Zazzo adds a fourth factor that is not taken into account in twin studies: the specific similarity created by the particular twin environment, or in other words, the twin situation. According to Zazzo, the twin situation generates both similarity in the behavior of MZ twins and dissimilarity in the depth of this similarity, i.e. The result of the twin situation is both convergence and divergence in the development of twins.

Thus, there is evidence of environmental differences between MZ and DZ twins. To reveal the special mechanisms that play a role in the development of MZ in contrast to DZ twins, it is necessary to consider those specific conditions, those elements of the environment with which the twin partners actively interact and are closely related [8]. Such environmental elements, as already noted, are, first of all, the specific relationship between parents and twins and the intrapair connection between partners in a twin pair. Let us consider data on some environmental conditions that may influence the similarities and differences between MZ and DZ twins.

RELATIONSHIP OF PARENTS WITH TWINS

Parents' interactions with their twin children have been studied primarily to test whether parents treat members of an MZ couple more equally than parents treat DZ twins. Additional data may also be obtained from studies in which parents of MZ and DZ twins rated their children's personality characteristics. The studies included mainly those families in which the twins were not of school age.

An analysis of the complex mechanisms of relationships that develop in the “parent-twin” team was carried out in the work of Allen et al. [10]. The study was carried out to find out how parents perceive and react to the objective differences and behavioral characteristics of each child in a twin pair. The authors include such differences, based on data from their early studies [9], [17]: ease of adaptation to new stimuli, level of excitability, difference in orientation towards objects (one twin is attracted to social contacts with people, the other to manipulations with objects ), the degree of confidence in one’s actions and the tendency to explore an object in one and the search for new stimulation and impressions in the other twin. Despite the striking similarities of some twins in MZ pairs, parents recognized even the smallest differences between them, attached special significance to them, and treated each member of the twin pair differently. The parents subtly encouraged and strengthened in the child that which corresponded to the role expected of each twin partner, and the child began to behave in accordance with this role. An important point, according to the authors, is the different position of MH parents in relation to the similarities of partners: some have an exaggerated denial of their similarities in a latent form (before the birth of children), and after the birth of twins, they emphasize the differences between them; others have a tendency to deny the difference between twins, avoid dangerous comparisons and emphasize their equality. It should be especially noted the specificity of parents’ attitude towards DZ twins. Of the 4 pairs in which intrapair differences were most pronounced, two were DZ (the study was conducted on 8 pairs of MZ and 2 pairs of DZ twins). The differentiation of DZ twins, who have obvious differences and a spirit of competition supported by their parents, is much more pronounced than in MZ twins.

A number of studies have examined the influence of twin similarity on parental treatment [18], [19], [27], [31]. Scarr's study [31] showed that family environment and parental treatment of twins were actually more similar for MZ than for DZ twins. Mothers rated their children on similarity in appearance, social maturation, early behavioral difficulties (feeding, hygiene skills, sleep, etc.). The ratings given to the MZ twins (23 pairs) were more similar than the ratings given to the DZ twins (29 pairs). Scarr put forward a hypothesis according to which it is not the parents' opinion about zygosity, but the genetic similarity of twins that determines the behavior of parents with their children. To test the hypothesis, the author analyzed the data of parents who had a false idea about the zygosity of twins, i.e. MZ were mistaken for DZ, and DZ twins were mistaken for MZ. It turned out that the parents' treatment of the twins corresponds to the true zygosity of the twins, and not to the parents' idea of it. In other words, greater similarity in upbringing is a function of the genetic similarity of twins, and therefore, according to Scarr, criticism of the twin method about the influence of the same developmental environment on the similarity of the results of MZ twins is unfounded. Confirmation of this point of view using more material was done in the works of Kozna et al. [18] and Lytton [27].

Cohen et al. studied parental behavior style in relation to twin pairs [18]. Parents of 181 MZ pairs and 196 DZ pairs of twins (102 DZ pairs were opposite-sex) participated in the experiments. The twins' ages ranged from 1 to 6 years. Data obtained using the authors' Parent's Report questionnaire showed that parents of MZ twins tend to treat their children more equally compared to parents of DZ twins. The discovered degree of similarity turned out to be unrelated to whether parents consider their children to be MZ, have a misconception about their zygosity (i.e. they mistook MZ for DZ twins, and there were 25 pairs of these) or are unsure of the zygosity of twins (MZ, in relation to of whom uncertainty was expressed, there were 26). Thus, the parents' understanding of the zygosity of twins did not influence the parental style of behavior in relation to twin children. Cohen et al. emphasizes that MZ twins, who are actually raised in relatively more identical conditions, are much more similar than DZ twins [19]. However, the authors note, the similarity of the MH partners may also depend on the fact that they are genetically identical and therefore tend to respond to specific environmental influences in the same way.

A study of the influence of the external similarity of twins on the development of their personality was carried out in the work of Plomin et al. [thirty]. The authors proceeded from the assumption that the significant similarity in appearance of MZ twins should lead to differences in the development of their personality due to the desire of partners to differ from each other and constant comparisons of members of a twin pair by people around them. Parents were asked to rate how often twin partners were mistaken for one another by parents (in early childhood and during the experiment), close friends, and casual acquaintances. Two studies were conducted: the first included 60 pairs of MZ and 35 pairs of DZ twins, the second included 60 pairs of MZ and 51 pairs of DZ twins. The twins' ages ranged from 2 to 6 years. The results of the study showed that MZ twins are more likely than DZ twins to be mistaken for one another. To determine the relationship between the confusion of twins in appearance and the similarity of their personal characteristics, Plomin et al. compared data from assessments of the external similarity of twin pairs and the results obtained from the EASI questionnaire for each pair [15]. Parents used this questionnaire to evaluate their children's emotionality, activity, sociability, and impulsiveness. It turned out that greater similarity in appearance does not make MZ twins more similar than DZ twins on measured personality traits. Based on the data obtained, the authors came to the conclusion that the unequal similarity in the appearance of MZ and DZ twins does not affect the magnitude of heritability estimates, at least when studying the personality of twin children.

Based on the research data, we can conclude:

parents treat MZ twins more equally than DZ twins;
members of MH couples are more often mistaken for one another compared to DH couples by parents, friends and acquaintances;
Despite the misconceptions of parents regarding the zygosity of twins, partners of twin pairs are treated in the same way as correctly classified MZ and DZ twins, i.e. according to their true zygosity;
greater similarity in appearance and parental attitude in MZ twins does not make twin partners (as assessed by parents) more similar compared to DZ twins in some personality characteristics.

The last two findings give grounds for some researchers to believe that the difference in environment for MZ and DZ twins is not a serious obstacle to the use of the twin method in the study of genetic and environmental determinants of psychological traits. At first glance, this contradicts the assumption of the influence of more equal parental attitudes towards MZ twins compared to DZ twins on the assessment of heritability in psychogenetic studies. However, most psychogenetic studies of personality use methods based on the self-esteem of the subjects, so an analysis of the characteristics of the development of self-esteem in twins is necessary. In the above works, the uniqueness of the relationship between parents and twins is studied in isolation from general psychological theories of personality and does not take into account such an important moment in the development of a child as the formation of his self-esteem.

In the process of developing a child’s personality, one of the central formations is the “I system”, and “... all further formation of the personality is closely connected with the development of self-awareness, which has its own specific characteristics at each age stage” [1; 34]. In the psychological sense, the term “Self-awareness” is usually used as the activity of a subject aimed at realizing his actions, experiences, and his place in the system of social relations [2]. The generalized results of self-consciousness and the emotional-value attitude towards oneself are consolidated into the corresponding self-esteem. All processes of the formation and development of self-awareness are indirect in nature, since they arise and develop in human activity and in his communication with other people [2]. The relationship between a child and an adult is a leading factor in the development of self-awareness under the age of 3 years. A stable self-esteem during this period has not yet developed, and “... the child spontaneously accepts the attitude of an adult towards himself, and, thus, the source of the initial self-esteem of his personality is the “on faith” accepted attitude of an adult” [6; 57]. Initial self-esteem also influences later personality development. An important point in the development of a child’s self-awareness is the process of awareness of oneself as a subject, which appears along with the use of the pronoun “I” [1]. Compared to single-born children, twins are delayed in using the pronoun “I” correctly. The preferred use of “we” instead of “I” is typical for MZ twins not only when one of the partners speaks on behalf of the couple, but also on his own behalf [37].

The amazing confusion of the pronouns “I” and “we”, characteristic of MZ twins, reflects not so much the difficulties in their mastering the grammatical forms of using these pronouns, but rather the special attitude of the people around them, and first of all their parents, who treat them as a couple, thereby influencing the formation of largely similar self-esteem among members of the MH couple. In this regard, one of the central issues when studying the environment of twins should be the study of the conditions for the development of self-esteem and the analysis of the specifics of its formation in MZ and DZ twins. An analysis of the relationship between parents and MZ twins allows us to make the assumption that the process of formation and development of self-esteem among members of an MZ pair is more identical than among members of a DZ pair, and this may be the reason for exaggerating the role of hereditary factors in psychogenetic studies of personality.

RELATIONSHIP BETWEEN TWINS

The uniqueness of the relationship of MZ twins with their parents is not the only point of criticism of the postulate of the twin method about the equality of the environment of MZ and DZ twins. The personality development of twins largely depends on the nature of the relationship between partners in the same twin pair. The unusual forms of communication between twins and each other can presumably be explained by the attitude of the people around them as a couple, as well as the result of the fact that two children who are at the same stage of physical and mental development, with rare exceptions, are constantly in each other’s company , have a common life experience and family environment. Joseph and Tabor [24] note that to understand the development of twins and the nature of twinning, it is necessary to consider the interaction between the “twinness response” and the desire of members of a twin pair to individuate. The “twinning reaction” is defined as the mutual identification of twin partners and the partial merging of the self-image (“I-image”) with the idea of an object (co-twin), which leads to “...diffusion of ego boundaries between these two people "[24; 277].

Von Bracken [13] identifies three points that are important in relation to twin partners:

the time the twins spend together (MZ twins, unlike DZ twins, are usually described as inseparable);
the desire of same-sex DZ twins for differences, competition, often observed among children in the same family, and the desire of MZ for “uniformity,” rarely found between ordinary brothers and sisters;
distribution of social roles in a couple, which is more common in MZ twins and remains more or less stable throughout life.

The concept of a “twin situation” is defined differently by different researchers, which largely depends on the purpose for which it is used by a particular author. Von Bracken [13] calls for the psychogenetic study of psychological traits to take into account the effect of the twin situation on the similarities and differences in the results of MZ and DZ twins. Zazzo [37], who most deeply analyzes in his work the influence of the twin situation on the development of the personality of twins, proposes to completely abandon the use of the classical twin method when studying the genetic foundations of human personality traits and use it to reveal the mechanisms of individualization of identical twins. In psychoanalytic work, attention is paid mainly to competition combined with interdependence and the formation of separate identities [12], as well as the influence of early experience on the further psychological development of twin partners [25], but these findings are not considered as limitations of the twin method.

The concept of a “twin situation” covers all those specific relationships that develop between members of the same twin pair. The common thing that is recognized in many works is the peculiar influence of the twin situation on similarities and differences, convergence and divergence in the development of the personality of twins. Thus, the twin situation can be one of the serious sources of errors when assessing the heritability of personality characteristics.

Differences between identical MZ twins are generated by the distribution of roles in the pair, the polarizing nature of which is one of the main factors in the individualization of partners. Von Bracken [13] identifies the following functions between members of a twin pair: representation in the outside world, when one of the twins: expresses the opinion of the couple {or the “foreign minister” of the couple); dominance in a pair, with the dominant twin having a decisive influence on his partner; and internal representation, in which one partner is the “conscience of the couple” (or the “minister of the interior” of the couple). Anastasi shares a similar point of view [11]. She highlights the role of the couple's representative in communicating with others, who has a greater interest in social contacts, and the role of the dominant twin, who tends to lead and make decisions for the two. According to von Bracken, the closer the connection between twin partners, the stronger the distribution of roles influences the personality development of twins, which affects MZ couples to a greater extent than DZ. The author suggests that role relationships between members of a twin pair have a polarizing effect on twins and are a source of error in psychogenetic studies of extraversion and in conflicting results in the study of dominance.

Leadership in twin pairs can take various forms, and the differentiated attitude of parents towards each child is important [10], [22], [34]. Shields [33], studying twins who grew up together, noted the connection between birth order, weight and height at the time of birth and during the examination period, and the degree of extroversion with the leadership of one of the twins in the pair. The leader tended to be born first, was heavier and taller, and scored higher on extraversion scales than his twin. Shields confirmed the idea that personality differences between MZ twins are largely due to their social interactions with each other. Denker [20] and Tienari [36] also came to a similar conclusion. Tienari points out that there is no connection between the role relationships that developed in childhood and later intrapair differences in twins.

The style of interaction between partners in twin pairs varies from pair to pair and affects interpersonal relationships with other people and the development of some personality traits of the twins themselves. Some data on the nature of interpersonal relationships in MZ twins were obtained by studying the dependence of the severity of neurotic symptoms on intrapair sociodynamics in a twin pair. Two opposing trends in the style of relationships between MZ twins were identified: distance from the partner, manifested in alienation, protests, the desire for independence from the co-twin, and the struggle for self-affirmation; and rapprochement with the partner, which is expressed in leadership and responsibility for the co-twin, in equality of members of the couple [23]. In accordance with two trends, Hagel-Ewers considers four forms of sociodynamics in twin pairs: 1) bilateral or one-sided rivalry, meaning the divergence of partners; 2) complementary subordination, the establishment of a socially meaningful distribution of functions, leading to the rapprochement of members of the couple; 3) symmetry and reciprocity of intra-pair relationships, leading to a balanced partnership; 4) bilateral alignment of Relationships, or conformity of the twin partners’ attitudes. The nature of the twins’ interaction with each other (the other was largely determined by the parents’ attitudes towards the pair and each twin separately. The highest degree of expression of neurotic traits (according to the Shepanck classification [32]) was found in twins with a tendency to isolate from each other, and the least - with a democratic style of interaction.

Coexistence gives the members of the MH twin pair properties and roles that are not initially predetermined by their early differences and do not depend on their identical heredity. The social environment plays an exceptional role in the sociodynamic interactions of twins. “Relationship with another” is impossible without some form of balance, which leads to the distribution of functions and complementarity of the partners’ positions [37]. Although minor differences in the physical characteristics and behavioral characteristics of MZ twins, present from the moment of their birth, may be the “material” basis for the future organization of the pair structure, they do not acquire the same significance outside of this structure. Zazzo draws attention to the specificity of the role relationships of MZ twins, in contrast to DZ, which manifests itself in variable leadership, when in some cases the leader is one twin, and in others his partner, and in the absence of clearly defined role relationships with exclusively harmonious connections between the members of the MZ pair .

The complex nature of the role interactions of MZ twins cannot be understood without analyzing the uniqueness of coexistence and the special conditions for the development of personality of the members of the MZ pair. Essential for the development of the personality of twins is the process of identifying oneself with a co-twin [29]. The initial relationship of twins is symbiotic in nature, in which each member of the pair is considered within a common boundary. This connection can last throughout the twins' lives. In this case, Geminis lag behind in the development of self-awareness; their personal contact with their mother and other people around them decreases. Ortmeyer [29] uses the concept of “We-Self” to describe “complementary, mutually shared aspects of personality” (p. 125), which means that the twin does not have a clear differentiation between his personality and the personality of the co-twin. The differences that exist between MZ twins can be leveled out by the process of their mutual identification. As Barlingham [14] suggested, the process of twins' identification with each other is based on similarities in emotional experience, and "makes identical twins 'identical' despite acquired differences" (p. 87). Constant contact with each other and a decrease in social ties with adults strengthen the mutual identification of the partners of a twin pair. The process of identifying twin children largely depends on the behavior of their parents. Parents often treat twins as a group, dress them in the same clothes, buy the same toys, and often mistake one twin for the other. Twins spend a lot of time together, since the mother cannot give them as much attention as she usually gives to one child. As children, twins usually sleep in the same bed and live in the same room. External similarity makes it possible for MZ twins, even in adulthood, to mystify those around them, passing one off as the other [21], [37]. The complementarity of intra-pair relationships leads to an increase in the effect of the twin situation and the interdependence of the twins, which affects their mental development. The twin “microclimate” influences the formation of mental functions in twin children. In MZ twins, there is a lag in the development of speech skills and the emergence of an intimate language that is understandable only to both of them, which leads to a delay in the development of their intellectual sphere [4], [37], [38].

Thus, the uniqueness of the twin situation leads to the specific development of MH twins, which can include difficulties in the formation of an “image of self” by each twin, intrapair dependence leading to complementary personality development, and leveling of differences due to the constant identification of twins with each other. All this may lead to similar development of some personality traits in MZ twins. On the other hand, role relationships between twins lead to divergent development of partners of a twin pair and can mask the influence of genotype on the development of certain psychological traits.

Analysis of the relationships between parents and twins and intra-pair connections between twin partners makes it possible to assume that the developmental environment of MZ twins differs from the environment of DZ twins. The specific connections of MZ twins with their parents and with each other can lead to convergent development of their personality and the formation of very similar self-esteem. Therefore, the peculiarities of the development of self-awareness in MZ twins must be taken into account in psychogenetic personality studies that use the twin method. The intrapair similarity of members of MH couples, traditionally interpreted as a result of the influence of hereditary factors, may in this case be due to the unique formation of self-esteem of MH partners.

In conclusion, it should be noted that in modern works in the field of psychogenetics, two lines of research are outlined: the first includes those in which the twin method is used to study the genetic and environmental determinants of behavior (the method of comparing intrapair similarity of MZ and DZ twins, the method of separated MZ twins, the control twin method and some others, less common), and to the second - works in which twin pairs are a special subject of research, an experimental model for studying the twin environment, as well as personality development, social relationships, the influence of parents on the development of children, etc. In At present, from our point of view, those psychogenetic studies that combine the study of genetic determinants of psychological traits with a deep analysis of environmental variables that influence their development are relevant. One of the possible options for such a study could be a thorough study of twins discordant for any characteristic of MZ [16]. Another way is also possible - when interpreting the results obtained by comparing data from MZ and DZ twins, take into account the well-known works on the unique development of MZ partners.

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History of twin studies in Russia

The first study of twins was conducted in 1900 by S. A. Sukhanov. He studied the features of the development of psychosis. Subsequently, in the 30s of the twentieth century, such research began to be substantially engaged in at the Medical-Biological Institute. This method has become key in the field of genetic psychophysiology. In 1972, in connection with this, the first laboratory was opened, where the twin method of genetics was central to the study of issues of genetic inheritance.

The main direction of research at the Medical Biological Institute was to study the role of the influence of genetics and environment on the etiology of diseases. Employees were directly involved in the study of diseases such as: bronchial asthma (N. N. Malkova), Graves' disease (I. A. Ryvkin), stomach and duodenal ulcers (A. E. Levin), diabetes (S. G. Levit) . Researchers at the institute obtained quite interesting data on the influence of heredity on the formation of psychophysiological characteristics in childhood (L. Ya. Bosik), features of the distribution of indicators on the electrocardiogram (I. B. Kabakov). Here, much attention was also paid to the development of a mathematical and statistical apparatus for analyzing the results using the twin research method (M. V. Ignatiev).

The institute conducted a study of the hereditary conditionality of cognitive functions (various types of memory, mental development, attention, intelligence). Mainly, researchers studied the features of active influence on a person through training. For this purpose, the control twin method was most often used, with the help of which the effectiveness of teaching methods was tested on another (experimental) subject (literacy, development of activity of a preschooler, and others). The results of such studies showed that training in repeating visual manipulations with objects did not give the expected effect on the speed of learning of preschool children and did not develop the activity of a preschool child.

Such domestic researchers as T. A. Panteleeva, I. V. Ravich-Shcherbo, G. A. Shibarovskaya and T. V. Vasilets studied differences in the properties of the nervous system of twins. The study of sonorant-motor reactions became the subject of research for T. A. Panteleev and S. B. Malykh. The study of psychological differences between twins was mainly carried out by V.V. Semenov, N.V. Iskoldsky and others.

Article on the topic “Twin method of studying the child’s psyche”

Twin method for studying the child's psyche

METHODS OF PSYCHOGENETICS (from the Greek psyche - soul, genos - origin) - methods that allow us to determine the influence of hereditary factors and the environment on the formation of certain mental characteristics of a person.

The most informative is the twin method. It is based on the fact that monozygotic (identical) twins have an identical genotype, dizygotic (fraternal) twins have a non-identical genotype; Moreover, members of twin pairs of any type must have a similar upbringing environment. Then the greater intrapair similarity of monozygotic twins compared to dizygotic twins may indicate the presence of hereditary influences on the variability of the trait being studied. A significant limitation of this method is that the similarity of the actual psychological characteristics of monozygotic twins may also have a non-genetic origin. As for the analysis of the heritability of normal psychological traits, this method, taken in isolation from other methods of psychogenetics, does not provide reliable information, because differences between populations in the distribution of a particular psychological trait can be caused by social reasons, customs, etc.

In child psychology, the study of V.S. is described in detail. Mukhina, in which the author studied her own children - twins, kept diary entries and analyzed the data. This method well reflects the influence of the environment on personality development, and also shows the influence of one’s own activity on mental development.

In experimental studies of child psychology, the “twin method” has a certain significance. As is known, identical twins have the same heredity. They represent, as it were, two copies of the same organism. In cases where it is necessary to check certain conditions and development possibilities, studies are carried out on twins.

The twin method is a comparative study of the conditions for the development and learning of twins. Its essence lies in comparing the characteristics of their development and the success of their assimilation of knowledge. Considering that monozygotic (single-ovular) twins have the same genetic code, the difference in their development and behavior gives grounds for the conclusion that the role of the influence of certain environmental factors, in particular educational factors, on the development of mental functions.

This method was used, for example, in the work of O.R. Luria and F.Ya. Yudovich. Researchers have been observing two twins for a long time who were noted to have significant delays in speech development. The results of the study showed that one of the factors that interferes with the development of full speech is that the communication of these twins with each other did not create an objective need for the development of correct, complete speech. Another factor was the tendency to have speech delays.

Then the researchers conducted a special formative experiment. They placed the twins in different kindergarten groups, thereby creating an objective need to communicate with other children, and also introduced an additional condition into the experiment: with one of the twins (the weaker) they began special language classes in order to develop better sound discrimination in the child, to develop a better pronunciation, etc.

As a result, data were obtained on the development of language and the entire mental development of twins under the influence of the need for communication; a difference was discovered in their language and thinking due to the fact that one of them studied systematically.

Main characteristics of the twin method

Twins are actually an ideal subject for psychogenetic research. The features of this method are mainly two things: the selection of subjects by comparing their genetic material (there must be the most exact match), as well as the central question of the study - the property that needs to be studied and differences found.

The first component is the most difficult to select. After all, there are two types of twins: monozygotic (MZ) and dizygotic (DZ). Monozygotic are identical and come from the same zygote (single common genetic material), while dizygotic are born from different zygotes (different genetic makeup). Monozygotic twins are less common than dizygotic twins. By comparing the results of intrapair correlations of these two categories, it is possible to directly determine the influence of the genotype on the developmental features of a particular trait being studied. However, to identify patterns of influence of the external environment on a trait, it is necessary to select a sample from monozygotic twins. This will make the study cleaner and avoid the influence of side factors on the results of the experiment.

The main problem of the twin method is the selection of subjects according to phenotypic indicators. In 1924, G. Siemens proposed using a method for assessing the level of zygosity - a polysymptomatic method of comparing the similarity of twins and assigning them to one or another group (mono- or dizygotic twins).

Main characteristics

Twins are the most “favorable model” for organizing psychogenetic research. There are two categories of twins :

identical or monozygotic,
dizygotic or dizygotic.

The twin method assumes that the study of the genotype and phenotype of twins allows us to identify and differentiate the degree of influence of heredity and environment on the formation of certain human characteristics.

Identical twins have the same genetic makeup because they are formed from the same zygote. That is why they are of the greatest interest to science. They are in some way clones of the same organism. However, monozygotic pairs are rare.

Fraternal twins are formed from different zygotes, so they can potentially develop different traits and characteristics.

If you create an individual environment for life and development , then you can monitor which features will develop regardless of external conditions (for each of the experimental subjects), and which are a consequence of unique conditions.

Those. a comparative assessment of the contribution of environmental and genetic prerequisites is carried out.

Basic evaluation criteria , concordance and discordance. If the trait appears in each twin of a pair (both have mental disorders), then we are talking about concordance for the trait.

If the symptom is observed in only one of the twins (one fell ill with schizophrenia, and the other remained healthy), then we are talking about discordance.

If, when comparing monozygotic (clones) and dizygotic twins, the concordance coefficient is approximately equal, then the researcher can assume that the feature was formed under the influence of non-genetic factors.

Features of the twin method

The twin method is used more and more often in science, so it is important to take into account the peculiarities of its implementation. In order to do this, it is necessary to take into account two main factors: the developmental environment and the lack of differences between twins. If you neglect the environmental factor, you can violate the internal validity of the study. For example, different behavior of parents towards twins can lead to false conclusions about the influence of the environment on the trait being studied (except if this is not the subject of the study). If we neglect the level of similarity between twins, this will affect the external validity of the study, since without establishing this factor, it will be impossible to know about the genetic component of the influence on the formation of the trait under study.

Sources of confounding effects in twin studies

The first source of error in such studies is the environment in which mono- and dizygotic twins are raised. It is usually different, since it has been established that the parents of such children treat them completely differently due to their external differences. Monozygotic twins look alike, so their parents treat them the same. But when raising dizygotic children, mom and dad try to note their differences. Therefore, this factor most influences the bias of the experimental results and must be controlled as a likely confounding variable.

A second source of error is the level of genetic similarity; this factor must be clearly defined before the experiment begins. Without checking it, you can get unreliable conclusions that will not correspond to the real picture of the study of scientific problems.

Mistakes in using the twin method

The essence of the first mistake is that the environment for raising mono- and dizygotic twins is significantly different due to the external differences of such children. Due to the fact that monozygotic twins look alike, their parents also treat them the same. Differences in the appearance of dizygotic twins also provoke different attitudes towards them on the part of close people. The second possible error is the level of genetic similarity, which should be clearly determined at the beginning of the study.

Classic method

This twin research method is carried out by comparing the results of the study of mono- and dizygotic twins. It is based on a comparison of intrapair correlations of these two types, taking into account the results of longitudinal observation (long-term study of the behavior of twins). Based on the results of this type of experiment, it is easier to establish reliable patterns in changes in the contribution of the environment and phenotype during the ontogenetic development of the subjects.

Features of the event

The twin research method is carried out in three stages :

Sampling.
Diagnosis of zygosity.
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.

Control twin method

This method is primarily used to study the influence of environmental determinants on a specific psychological trait. It can only be used if exact similarity between two monozygotic twins is determined. These pairs are divided, where the first is assigned to the experimental group, and the second to the control group. Further, after completion of the experiment, significant differences in the influence on the experimental sample are established when compared with the indicators in the control sample. If such differences are found, then the effectiveness of the experiment is reliable.

Separated twins method

The most brutal twin method in psychology. The object of such research is mono- and dizygotic twins who were separated in early childhood and do not know about each other’s existence. The similarity of such children is due only to the genotype, and the environment and its influence are completely different. Such cases are extremely rare, so there are only a few such studies. Here it is impossible to control the influence of secondary variables, and the sample may not always be sufficient to draw reliable conclusions.

6. Comparison of twins with non-twins.

Also an auxiliary method that allows you 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 was noted.

single borns. This difference is especially noticeable at an early age. But

comparison of the results of a study of members of twin pairs whose partner

died in early childhood and single births did not reveal a significant difference in

level of development. That is, the developmental features of twins are not due to

as much with the difficulties of embryonic development as with the peculiarities

raising twins as a couple (family difficulties during the birth of twins,

isolation of twins in a pair, etc.). So the twins are somewhat

differ from the entire population, but with age this difference is noticeably smoothed out

and twins, for the most part, become comparable to the rest of the population.

Twin family method

It combines family and twin types of research. Most often used in studying the influence of heredity on the development of mental illness. The twin method of studying human heredity is aimed at studying family members of adult twins and their children. Thanks to this method, it is possible to study the role of “maternal” or “paternal” influence effects on the psychological development of non-twin children.

Single twin method

The object of this method of study is single twins who survived as a single number after childbirth. It is good for identifying the role of perinatal development on the subsequent ontogenetic process of development of the child’s personality. This twin research method is carried out through observation and helps to trace the psychological characteristics of the “twin situation” and observe the influence of other determinants. Used as a complement to the classic twin method.

Advantages of the twin method

There are many scientific positions regarding the effectiveness of the twin method. Some researchers are inclined to consider it invalid, but the history of its use in practice speaks for itself. The combination of all psychophysiological methods together with this makes it possible to solve a fairly wide range of issues. Therefore, the twin method is used in the field of general, differential, developmental, and educational psychology. It is also used in many other industries that are not related to psychological research (genetics, physiology, biology, and others). The twin research method, despite criticism, is widely used in many areas of science and allows us to identify individual psychological differences and key determinants of psychological development of the individual.