TWINS. — Twins are said to be the children born in the same birth and from the eggs of the same deposition. In particular cases, several individuals may be born even from a single egg; thus, one distinguishes between polyovular and monovular twins; in the case of mammals, where the egg is enclosed in the chorion, one speaks of mono-chorial and pluri-chorial twins (in the case of double twinning, bi-chorial).
Polyovular twins may also be of different sexes, whereas monovular twins are always of the same sex and, in the human species, are extremely similar, often with mirror-like resemblance. 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 various animals; the human species is generally uniparous and exceptionally pluriparous (from 1 to 6). According to statistics derived from Chiarugi, the percentage of multiple births in the human species is:
- in the case of double twinning, one in 84.6 simple births;
- in the case of quintuple twinning, one in 23,608.5 simple births.
In cases of monovular twinning, one speaks of polyembryony. This is a normal phenomenon in some animals, such as, for example, among mammals in the *Tatusia* (an armadillo) with 4–12 twins. In certain insects, thousands of individuals can originate from a single egg (cases studied by Marschall and Silvestri in Hymenoptera).
Polyembryony has also been studied experimentally in amphibians. If the first two blastomeres of a cleaving egg or two groups of blastomeres are separated, two distinct individuals are formed. In invertebrates (rotifers), as many as 16 embryos have been obtained. These studies have demonstrated the origin of double monstrosities (Siamese twins, bicephalic monsters, and other teratological conditions) by provoking incomplete separations between the blastomeres of the egg. The problem of individuality in relation to these facts has been discussed elsewhere (v. ANIMATION).
The tendency toward twin births in women depends on the mother’s constitution and is therefore hereditary.
PSYCHOLOGY. — The question of twins
concluded that the influence of the environment is scarce on intelligence, feeling, and will. It can be studied from numerous points of view: historical, iconographic, in its causal factors, in the embryogenetic mechanism, in the manifestations and mutual relationships of life among the members of the small twin society, between them and the society that receives them; in particular, twins can be studied with regard to the manifestations of their psychic life, both to observe any characteristic behavior and to use them as test subjects for experimental psychology, for which the fortunate combination of perfectly similar individuals on whom different causes can be made to act may serve.
The first experimental psychological study of twins is due to Galton (1882), who highlighted the enormous similarity of the psyche of monozygotic (MZ) or monovular twins, i.e., those born from the same egg. Out of 80 pairs of twins so similar as to be mistaken for each other, Galton found that in 11 cases they made the same remarks on the same occasions, started singing the same song at the same time, and so on; one would begin a sentence, the other would finish it...
In 1905, Thorndike introduced into the psychological study of twins the system of mental tests, conducting his research on a group of 50 pairs of twins drawn from the school population of New York. From his research, he concluded that the resemblance of twins does not increase with age; on the contrary, it decreases; that it is always much less (about half the coefficient) even in siblings separated by only a few years (3–4); and that the resemblance is much closer with regard to those aptitudes most exercised in school life. From this, he concluded that the facts find explanation in the predominant influence of the germ cells, hence of heredity.
Antebrach (1925) studied 212 pairs of twins, not only from a biological but also from a psychological point of view, using various tests (intelligence quotients, arithmetic, memory, handwriting, specific skills, etc.); he was thus able to confirm that the correlation is higher among twins than among single-born siblings, and higher among same-sex twins than among those of different sexes. With regard to handwriting, he found only in a few cases such similarity as to prevent individualization.
Halton (1926) observed a pair (MZ) of 8 years, first observed in different environments and then reunited, noting discrepancies in character that later disappeared, thus emphasizing the influence of the environment on psychic characteristics.
In 1931, Lortig, from studies of 10 pairs of twins, concluded that the influence of the environment is slight. 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, so that he did not doubt that hereditary factors are the determining moment.
Subsequent studies, always on pairs of twins with regard to psychic faculties, conducted by Lenz (1932), Kuhlmann (1932), Miercke (1934), Brecken (1936), and others, are noteworthy for their importance. In 1950, Gedda and Miglio studied the attitudes of MZ twins toward clothing in 12 pairs aged between 7 and 20, concluding that MZ twins, although physically so similar as to be able to exchange clothes, in the vast majority of cases (91.9%) desire to wear identical clothes, do not indiscriminately use the other’s or their own garment, and in the majority of cases (87.05%) prefer their own, which is quite recognizable.
Important elements for the study of the psychic similarities of twins can be drawn from the study of their handwriting, which, in a certain percentage of cases not very high, is so similar as to be indistinguishable even by experts; moreover, it seems that the similarity of handwriting in MZ co-twins becomes even greater with age after 14 years.
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 each other in an extraordinary way. One may recall the example of Johann Ambrosius Bach (father of the famous Johann Sebastian) and Johann Christoph Bach, co-twins, both famous musicians, perfectly similar, so that even their wives were unable to distinguish them, identical in play, pronunciation, thought, and artistic manifestations; they fell ill and died almost simultaneously, one after the other. Other twins may be recalled in whom life unfolded in a very similar manner in external social manifestations; thus in Augusto and Giovanni Picard, famous for their stratospheric careers, and in many others, of whom it is not possible to speak at length, but who, even if circumstances led them to be separated, nevertheless had in common a similar curriculum vitae. Numerous reports of this kind highlight the importance of the psychological study of twins through the global evaluation of their lives; such a study can reveal the real aptitudes of twins and the existence of their concordance, so frequent in MZ twins, much better than a merely sporadic morphological comparison; the dynamics of their life within the social community allows for the establishment of a psychological profile of far greater value, which often escapes those who limit their examination to test batteries.
The psychology of twins can be studied from three particular points of view: 1) from the perspective of the single individual that constitutes the group of the twin society, in comparison between the psyche of the twin and that of the single-born; a) in relation to
Conditions created by coexistence within the tiny “intrageminal society,” in their internal and external relations; 3) as an experimental means of analogy for the general study of the human psyche.
Regarding the first point, the most important conclusions concerning the psychology of twins can be summarized in four laws: a) the particular psychological situation determined by twin conception does not, in absolute terms, diminish the psychic efficiency of the twin; b) the monozygotic twin possesses a personal psychic individuality of his own; c) the psychic individuality of the monozygotic twin in relation to his co-twin is not always proportional to physical individuality; d) various authors postulate different mechanisms as determining intrageminal differences that distinguish the monozygotic twin (embryogenetic, psychic, etc.).
Regarding the second point, that is, social psychology which unites individual co-twins in an intrageminal society, the conclusion is reached that: a) the members of each twin pair constitute a tiny society that spontaneously manifests itself on a natural foundation, producing a characteristic psychological sphere which interposes itself between the individual psychological sphere and other family psychological spheres, thereby giving rise to a true “intrageminal society”; b) the intrageminal society presents, from a psychological standpoint, two varieties: monozygotic and polyzygotic; c) the intrageminal society is not uniform but achieves a psychological differentiation of intrinsic and extrinsic society (differentiation of roles between a twin who represents the society externally and one who commands within the intrageminal society); d) conflicts between members of the intrageminal society obey, so to speak, the law of all or nothing—trivial or, very rarely, extremely serious; e) the intrageminal society possesses characteristic customs and usages (hermetic language, etc.).
Regarding the third point, the study and experimentation of twin psychology offers the possibility of evaluating how much in the human psyche is hereditary and how much is to be attributed to the environment. On this subject, the following conclusions are presented: a) heredity also plays a highly significant role in the psychic sphere; b) hereditary determinism of psychic activities varies in degree according to the functions considered; hereditary influence is felt with decreasing value on intelligence, feeling, will, and temperament; c) the fourth dimension, namely time, brings about modifications in individual psyche, which is distinguished by successive different equilibria in the sense that the polygon of active forces (heredity, environment) records, over time, a relative attenuation of the hereditary component and a relative increase in the environmental component; d) not only general psychology but also psychology applied to education is concerned with taking account of constitutional psychic qualities in order to evaluate the most effective methods; by using the twin society, the method has been employed (America, Russia) by influencing only one member with the means under examination while keeping the other as a control; e) different psychic characters prove to be strongly dependent on hereditary constitution rather than on environment; f) hereditary determinism of the human psyche is not absolute, that is, although heredity provides the building blocks for the construction of individual personality, this is also subject to other causal movements, among which free will is of primary importance.
The question of twins also opens the field to the study of important philosophical and theological problems and contributes to the resolution of grave doctrinal controversies.
From ancient times, philosophy and theology have concerned themselves with twins; thus Diogenes, Cicero, St. Augustine, St. Gregory, etc., often with concepts that were highly imprecise from a biological standpoint.
The study of twins can provide arguments of considerable importance to the problem of animation (v.) in the debated question of the immediate influence of the rational soul at the moment of conception of the new individual, or rather of the mediated or delayed influence at the moment when the body has reached that degree of organization sufficient to render it, at least potentially, suited to an intellectual life. This Thomistic philosophical concept follows the *Settimane*, and to this thesis of delayed influence Monsignor A. Lanza adhered 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 episcopal champion expressed himself in these terms: “It may therefore be considered, with some probability, that at the end of the embryonic period, which for man lasts about eight weeks, when the process of growth has already decisively prevailed over the processes of formation, the body is sufficiently organized to receive the rational soul; unless a teratological process has arrested or otherwise deformed its development... If the human soul is infused by God into the body only after it has reached, in its organization, that degree of development which befits it in order to be considered specifically perfect, it is evident that whenever, for any reason, teratological processes arrest this development or otherwise influence it, the result will be an organism radically unfit for the functions of the species; the individual thus resulting will not be informed by the rational soul. Any doubt that may still remain will be, for the most part, of a physiological nature and will concern the degree of vitality of the embryo and its functional independence, where the case is one of double monstrosity.”