Twins

TWINS. - Twins are children born in the same birth and from the eggs of a single laying. In particular cases more than one individual can be born even from a single egg; a distinction is therefore made between multi-ovular and monovular twins; in the case of mammals, in which the egg is enveloped by the chorion, one speaks of monocorial and pluricorial twins (in the case of double twinning, bicorial). Multi-ovular twins can also be of different sex, whereas monovular twins are always of the same sex and, in the human species, are extremely similar, often with mirror-image similarity. This is due to the fact that, deriving from the same egg, they have the same hereditary endowment.

The condition of uniparous and pluriparous offspring is specific to the various animals; the human species is generally uniparous and exceptionally pluriparous (from 2 to 5 twins). According to statistics drawn from Chiarugi, the percentage of multiple births in the human species is: in double twinning, one in 84.6 single births; in triple twinning, one in 6,731 single births;
in quadruple twinning, one in 963,612 single births; in quintuple twinning, one in 23,608,502 single births.

In cases of monovular twinning one speaks of polyembryony. This is a normal phenomenon in certain animals, as, for example, among mammals in the tatuzia (an armadillo) with 4-12 twins. In certain insects thousands of individuals can originate from a single egg (cases studied by Marschall and by Silvestri in the hymenoptera).

Polyembryony has also been studied experimentally in amphibians. If the first two blastomeres of an egg in segmentation are separated, or two groups of blastomeres, two distinct individuals are formed. In invertebrates (jellyfish) as many as 16 embryos have been obtained. With these researches it has been possible to demonstrate the origin of double monstrosities (Siamese twins, two-headed monsters and other teratological conditions) by provoking incomplete separations between the blastomeres of the egg. The problem of individuality in connection with these facts has been discussed elsewhere (v. ANIMAZIONE).

The tendency to twin births in women depends on the mother's constitution and is therefore hereditary in nature.

Alberto Stefanelli

PSYCHOLOGY.

The question of twins can be studied from numerous points of view: historical, iconographic, in its causal factors, in the embryogenetic mechanism, in the manifestations and in the mutual relations of life among the members of the small twin society, among themselves and with the society that receives them; in particular, twins can be studied with regard to the manifestations of their psychic life, both to see in them a possible characteristic behavior and to avail oneself of them as subjects for the study of experimental psychology, for which the fortunate combination of perfectly similar individuals on whom different causes may be made to act can serve.

The first study of experimental psychology on twins is due to Galton (1883), who brought out the enormous similarity of the psyche of monozygotic (MZ) or monovular twins, that is, born from the same egg. Out of 80 pairs of twins, so similar as to be mistaken for one another, Galton finds that in 11 cases «they make the same remarks on the same occasions, they begin to sing the same song at the same moment, and so on; one begins a sentence, the other finishes it...».

In 1905 Thorndike introduced into the psychological study of twins the system of mental tests, carrying out his researches on a group of 50 pairs of twins drawn from the school population of New York. From his researches he reached the conclusion that the resemblance of twins does not increase with age, but rather diminishes; that it is always much smaller (about half the coefficient) in siblings even if separated from one another by a few years (3-4); that the resemblance is much closer with regard to those aptitudes more particularly exercised in school life. From this he concluded that the facts find their explanation in the predominant influence of the germ cells, hence of heredity.

Lanterbach (1925) studied 212 pairs of twins not only from the biological point of view but also from the psychological one, by means of various tests (tests of intelligence, of arithmetic, of handwriting, of specific abilities, etc.); he was thus able to confirm that the correlation is higher in twins than in non-twin siblings, and higher in twins of the same sex than in those of different sex. With regard to handwriting, only in a few cases did he find an equality such as not to permit individualization.

L'Halon (1926) observed in a pair (MZ) of 8 years, who had first lived in a different environment and were then reunited, discordances in character which later disappeared, and therefore insisted on the influence of the environment on psychic characteristics.

In 1931 Lortig, from studies of 10 twin pairs, concluded that the influence of the environment is slight and that intelligence, feeling and will are determined by heredity.

In 1931 Looft, studying the intelligence of twins, noted that mental retardation is concordant in co-twins, and therefore did not doubt that hereditary factors are the determining element.

Worthy of note for their importance are the subsequent studies conducted, again on pairs of twins, with regard to the psychic faculties, by Lenz (1932), Kuhlmann (1932), Miercke (1934), Brecken (1936) and still others. In 1950 Gedda and Miglio studied the attitude of MZ twins with regard to clothing in 112 pairs between the ages of 7 and 20, concluding that MZ twins, although physically so similar as to be able to exchange clothes, and although desiring in almost all cases (91.9%) to wear identical clothes, do not use the other's garment or their own indifferently, preferring for the most part (87.05%) their own, quite recognizable by

(phot. Orlandini)

GEMINIANO, saint - Miniature of the Relatio translationis corporis 1. Geminiani (13th cent.) depicting the discovery of the Saint's sarcophagus, in the presence of Countess Matilda - Modena, Archivio capitolare.

small marks that escape strangers.

Important elements for the study of the psychic resemblances of twins can be drawn from the behavior of their handwriting which, in a certain not very high percentage of cases, is similar to the point of not being distinguishable even by experts; it would further appear that the calligraphic resemblance in MZ co-twins becomes ever greater with growth, after 14 years of age.

A further evaluation of the psychological similarity of twins can be drawn from their "curriculum vitae." Thus the careers of MZ twins are seen to resemble one another in an extraordinary way. One may recall the example of Johann Ambrosius Bach (father of the famous Johann Sebastian) and of Johann Christoph Bach, twins, both famous musicians, perfectly alike, so that even their own wives were unable to distinguish them, identical in play, in pronunciation, in thought, in artistic expression, who at the same time loved one another and died soon, one after the other. Other twins have been recorded in whom life unfolded most similarly in its external social manifestations; thus in Augustus and John Picard, famous for their stratospheric career, and in many others, of whom we cannot treat at length, but who, even if the circumstances of life brought them to be separated, nevertheless had in common a similar "curriculum vitae." Numerous reports of this kind bring out the importance of the psychological study of twins through the global evaluation of their life; such a study can highlight the real aptitudes of twins and the existence of their concordance, so frequent in MZ twins, much better than a merely morphological comparison sporadically carried out can; the dynamics of their life within the social community makes it possible to establish a psychological profile of far greater value, and one that often escapes those who limit their examination to batteries of tests.

The psychology of twins can be studied from three particular points of view: 1) from the side of the single individual entity that constitutes the group of the twin society, in a comparison between the psyche of the twin and that of the singleton; 2) in relation to the particular conditions created by living together in the tiny "intrageminal society," in its internal and external relationships; 3) as an experimental means of analogy for the generic study of the human psyche.

With regard to the first point, the most important conclusions regarding the psychology of twins can be summarized in four laws: a) the particular psychological situation determined by twin conception does not diminish, from the absolute point of view, the psychic efficiency of the twin as a human being; b) the MZ twin possesses a psychic individuality of his own; c) the psychic individuality of the MZ twin with respect to his co-twin is not always proportional to physical individuality; d) different mechanisms are postulated by various authors as determining the intrageminal differences that individualize the MZ twin (embryogenetic, psychic, etc.).

With regard to the second point, that is, the social psychology that unites the individual co-twins in an intrageminal society, one reaches the conclusion that: a) the members of every twin pair constitute a tiny society that spontaneously manifests itself on a natural foundation and that produces a characteristic psychological sphere, which interposes itself between the individual psychological sphere and the other family psychological spheres, thus giving rise to a true "intrageminal society"; b) the intrageminal society offers, from the psychological point of view, two varieties: the monogistic and the polyzygistic; c) the intrageminal society is not uniform, but brings about a psychological differentiation of the intrinsic and extrinsic society (differentiation of tasks between one twin who is the representative toward the outside and one who is the leader within the intrageminal society); d) conflicts between the members of the intrageminal society obey, so to speak, the law of all or nothing, being futile or, very rarely, extremely serious; e) the intrageminal society possesses characteristic customs and usages (hermetic language, etc.).

With regard to the third point, the study and experimentation of twin psychology offers the possibility of evaluating how much is hereditary in the human psyche and how much is to be referred to the environment. Regarding this subject the following conclusions are set forth: a) heredity assumes a part of great importance in the psychic sector as well; b) the hereditary determinism of psychic activities is more or less great depending on the functions considered; hereditary influence makes itself felt with a scale of decreasing value on intelligence, feeling, will, temperament; c) the fourth dimension, that is, time, determines modifications in the individual psyche, which appears marked by various successive equilibria in the sense that the polygon of active forces (heredity; environment) registers, with time, a relative attenuation of the hereditary component and a relative increase of the environmental component; d) not only general psychology, but also psychology applied to education, is interested in taking account of constitutional psychic qualities, in order to evaluate the most effective methods; using the twin society, the method has been used (America, Russia), influencing with the means under examination only one member, keeping the other as a control; e) different psychic characters prove to be strongly tributary to the hereditary framework, that is, to constitution; f) the hereditary determinism of the human psyche is not absolute, that is, although heredity provides the building stones of the edifice of the individual personality, this is also subject to other causal moments, among which principally free will.

The question of twins moreover opens the field to the study of important problems of a philosophical and theological order, and contributes to the solutions of grave doctrinal controversies.

Since the past, philosophy and theology have occupied themselves with twins; thus Diogenes, Cicero, St. Augustine, St. Gregory, etc., often with concepts that were anything but precise from the biological point of view.

The study of twins may bring arguments of notable importance to the problem of animation (v.) in the debated question of the immediate infusion of the rational soul at the moment of conception of the new individual, or rather of the mediated or delayed infusion at the moment when the body has reached that degree of organization such as to render it, at least in potency, suited to an intellectual life. This concept of Thomist philosophy follows Sertillanges, and this thesis of delayed infusion was adhered to by the recently deceased Msgr. A. Lanza in a monograph (La questione del momento in cui l'anima razionale è infusa nel corpo) published in the Bollettino filosofico (4 [1938], pp. 211 ff.; 335 ff.; 5 [1939], pp. 36 ff.; 206 ff.). The lamented bishop expressed himself in these terms: "It may be held, therefore, with some probability, that at the end of the embryonic period, which for man lasts about eight weeks, when the process of growth already acquires a decided prevalence over the processes of formation, the body is sufficiently organized to receive the rational soul; unless a teratological process has arrested or in any way deformed the development... If the human soul is infused by God into the body only after this has reached, in its organization, that degree of development that suits it so that it may be called specifically perfect, it is evident that every time, for whatever reason, teratological processes arrest this development or influence it in any way, so as to make it terminate in an organism radically unfit for the functions of the species, the individual that results will not be informed by the rational soul. If doubt, at times, may still subsist, it will be predominantly of a physiological nature and will have as its object the degree of vitality of the embryo and its functional independence, where double monstrosity is concerned."