Because of the identical genes carried by monozygotic twins, they have special appeal scientists for use in medical and psychological studies. Sets of MZ and DZ twins (the control set) are matched to each other and compared. Using this "twin method" scientists attempt to shed some light on the nature or nurture theories of development of certain diseases, the formation of personality and intelligence, as well as the development of schizophrenia, eating disorders, depression and susceptibility to infectious diseases. Studies have shown that there is a strong genetic link to the development of the psychological disturbances and drug dependancies, since so many MZ twins share such conditions. This has aided the studies of the relative importance between heredity and environment in an individual's development. For more on twins reared apart and reunited later in life, click here.
(from Psychotherapy and Psychosomatics journal)
This study attempted to find which is more important in the etiology of anorexia nervosa, heredity or environment. The subjects included 7 sets of identical twins, all female, who showed varying degrees of the condition. Only one set displayed all criteria for anorexia nervosa. Usually one twin developed the condition and influenced or pressured the other to follow suit. When separated to prevent competitive behavior, the twins resumed normal eating patterns. Monozygotic twins were also more concordant to the condition than dizygotic twins. The frequency of concordant cases of anorexia nervosa was higher in MZ twins in other studies as well. These kinds of results suggested that it is hightly probable that a hereditary componant is involved in the disturbance. However, the variance in the degrees of anorexia development among the twins led the scientists to hold off on a result before more precise quality of the condition could be met. (It would seem to me the very close nature of the identical twin relationship would compel twins to compete more than normal siblings, thus the frequency of this competition to be thin. This happened to twins I know.)
(from Archives of General Psychiatry)
"Suicide appears to cluster in families, suggesting that genetic factors may play a role in this behavior. We studied 176 twin pairs in which one or both twins had committed suicide. Seven of the 62 MZ twin pairs were concordant for suicide compared with two of the 114 DZ pairs (11.3% vs 1.8%). The presence of the psychiatric disorder in the twins and their families was examined in a subsample of 11 twin pairs, two of whom were concordant for suicide. Eleven of these 13 twin suicide victims had been treated for psychiatric disorders, as had had eight of their nine surviving cotwins. In addition, twins in 10 pairs had other first or second degree relatives who had been treated for psychiatric disorder. Thus, these twin data suggest that genetic factors related to suicide may largely represent a genetic predisposition for the psychiatric disorders associated with suicide. However, they leave open the question of whether there may be an independant genetic componant for suicide."
(from Acta Geneticae Medicae et Gemellologiae Twin Research )
Twins are reported to invent their own shared languages which are unintelligible to others. These automomous languages "exist in 40% of all twins, but often disappear soon." The language consists of onomatopoeic expressions, invented words and words from the adult language "adopted to the constrained phonological possibilities of young children." Twins are able to understand and communicate in this automomous language because their language development patterns are similar and they reinforce each other's "deviant articulation" by repeating and maintaining mistakes made in word formation. The phonological distortions in these autonomous languages are not exactly predictable, and often sounds which do not exist in the adult language are produced. The languages of the 9 twin pairs studied consisted of nouns and verbs strung together, and the vocabulary imitated that of the parents, but not the sentence structure. Most children who developed autonomous languages were left to themselves, had little contact with other children during their early years. Also, their parents could have provided a poor language model by not correcting their pronunciation mistakes. The study concludes that environmental factors, namely the twin relationship itself are most important to the development of autonomous languages. The development of twin languages occurs more often in MZ twins than in DZ twins because "the psychological bond between DZ twins is less intense than between MZ twins." However, the study did find that autonomous languages are not restricted to twins--triplets, siblings close in age or even close friends-- nor are they invented or intended to be secret. It was sometimes reported that children were upset that their parents could not understand their language.
How often have I heard the question, "Do you and your sister ever get yourselves mixed up?" The answer is no, not ever, of course not. And here's a study that suggests that identical twins never confuse their identities from each other, starting at a very early age.
(from Australian Journal of Psychology)
Toddler monozygotic twins were shown a variety of photographs of other young children and their reactions were monitered. Each child viewed the pictures independantly from his or her twin. Reactions ranged from visually fixated staring to smiling to "coy." The response patterns suggested that twins had already formed an ability to discriminate between an image of themselves and one of their "mirror image" twin. "A greater incidence of embarrassed, self-conscious (and coy) behaviour occurred to the self than to the twin image." It's possible that the cohesive bond of twins and their preference for interaction with each other rather than with other children may promote more discriminating awareness of features which distinguish one from the other. For more information on identity and intimacy between twins, click here.
(from Twins; Nature's Amazing Mystery) Twins often share phobias, and more often than not if one twin harbors an irrational fear, the other does too, if her childhood was not marred by a traumatic experience. Phobias are thought to be caused by traumas, but the Minnesota studies on twins offered contrary data. Twins who had been reared apart and then reunited later in life shared alike phobias, and some phobias were pretty obscure: One pair were fearful of escalators, and would not set foot on one. One twin might be less frightful than the other, but usually both shared the phobia. It is thought that phobias common among twins, claustrophobia (fear of closed in places) and agoraphobia (fear of open spaces), might be derived from the closeness of the space twins first share--the womb!