Levels of Identity Confusion and Attachment among Reared-Together MZ and DZ Twin Pairs

(A lightly edited version of an online article originally published on 4-21-2020)

When twin researchers attempt to assess twins’ environmental similarity, they usually ask questions such as whether, as children, twins shared the same bedroom, attended school together, dressed alike, played together, and so on. Answers to these questions reveal that reared-together MZ twin pairs (monozygotic, identical) grow up experiencing much more similar environments than experienced by same-sex DZ twin pairs (dizygotic, fraternal). Although these questions address some aspects of twins’ environmental similarity, they fail to adequately assess the nature of the attachment, conscious attempts to be alike, and the identity confusion experienced by MZ twin pairs to a far greater degree than DZ pairs.

In 1960, family therapy pioneer and twin study critic Don Jackson described “the intertwining of [MZ] twin identities, in the ego fusion that in one sense doubles the ego (because the other is felt as part of the self) and in another sense halves it (because the self is felt as part of the other).” According to the psychoanalytically oriented twin researcher Dorothy Burlingham, “Identical twins when they grow up often fail to develop into separate human entities” (quoted in Jackson). And in their field-defining 1960 book Behavior Genetics, John Fuller and William Thompson recognized that “MZ cotwins model their behavior upon each other to a greater extent than DZ cotwins.”

The table I present below provides data from all twin studies I am aware of that published percentage figures (or enough information to calculate percentages) relating directly to the levels of identity confusion and attachment that twins experience. We see that in a 1954 study, British twin researcher James Shields found that 47% of the MZ pairs experienced a “degree of attachment” that was “very close,”. In contrast, only 15% of the DZs experienced a very close degree of attachment. Swedish researcher Torsten Husén calculated an “index of attachment” for twins and found “a considerable mean difference” between MZ and DZ pairs. Husén concluded in 1959 that MZ pairs “are much more prone to emphasize the desire to be alike, to be together, to share the same interests, and to have a feeling of loyalty.” In 1967, Norwegian twin researcher Einar Kringlen performed a “global evaluation of twin closeness,” and found that 65% of the MZ pairs had an “extremely strong level of closeness,” which was true for only 17% of the DZ pairs. And in a 1966 twin study, Helen Koch of the University of Texas found that “Identical [MZ] co-twins tended to be closer to each other than fraternals [DZs].”

Some results in the table are taken from twin researchers’ use and development of zygosity determination questionnaires, which ask twins to answer questions to distinguish MZ from DZ pairs for research and other purposes. Although there are various methodological issues in the studies shown in the table, the trend is clear that MZ pairs experience much higher levels of identity confusion and attachment than experienced by DZ pairs, which argues strongly against the twin method’s crucial MZ-DZ “equal environments assumption” (EEA).

Of the researchers using or developing zygosity determination questionnaires (marked by an asterisk), only the Cohen group commented on the irony of needing to demonstrate the great dissimilarity of MZ and DZ childhood environments to distinguish between such pairs. The irony is that the twin method assumes that these environments are not dissimilar. According to Cohen and colleagues, “The impact of such repeated confusion on individual twinships, or the effect of these differences between MZ and DZ twins is not known with certainty. However, such information must cast doubt upon the assumption of environmental equivalence [EEA]” (Dibble et al.,1978).

The table below is adapted from my 2015 book The Trouble with Twin Studies: A Reassessment of Twin Research in the Social and Behavioral Sciences, New York: Routledge (pp. 166-167).

© 2015 Jay Joseph. An earlier version of this table appeared in a 2013 article I published in The Journal of Mind and Behavior.

I ask twin researchers and their supporters to explain how the results in this table are consistent with the claim that the twin method’s MZ-DZ “equal environments assumption” is valid.

REFERENCES

Cederlöf, R., Friberg, L., Jonsson, E., & Kaij, L. (1961). The diagnosis of twin zygosity. Acta Genetica et Statistica Medica, 11, 338-362.

Cohen, D. J., Dibble, E., Grawe, J. M., & Pollin, W. (1973). Separating identical from fraternal twins. Archives of General Psychiatry, 29, 465-469.

Cohen, D. J., Dibble, E., Grawe, J., & Pollin, W. (1975). Reliably separating identical from fraternal twins. Archives of General Psychiatry, 32, 1371-1375.

Dalgard, O. S., & Kringlen, E. (1976). A Norwegian twin study of criminality. British Journal of Criminology, 16, 213-232.

Dibble, E., Cohen, D. J., & Grawe, J. M. (1978). “Methodological issues in twin research: The assumption of environmental equivalence.” In W. Nance (Ed.), Twin research: Psychology and methodology (pp. 245-251). New York: Alan R. Liss.

Fuller, J. L., & Thompson, W. R. (1960). Behavior genetics. New York: John Wiley & Sons.

Husén, T. (1959). Psychological twin research: A methodological study. Stockholm: Almqvist & Wiksell.

Jackson, D. D., (1960). “A critique of the literature on the genetics of schizophrenia.” In D. Jackson (Ed.), The etiology of schizophrenia (pp. 37-87). New York: Basic Books.

Joseph, J. (2013). The use of the classical twin method in the social and behavioral sciences: The fallacy continues. Journal of Mind and Behavior, 34, 1-39.

Kasriel, J., & Eaves, L. (1976). The zygosity of twins: Further evidence on the agreement between diagnosis by blood groups and written questionnaires. Journal of Biosocial Science, 8, 263-266.

Koch, H. L. (1966). Twins and twin relations. Chicago: The University of Chicago Press.

Kringlen, E. (1967). Heredity and environment in the functional psychoses: An epidemiological-clinical study. Oslo: Universitetsforlaget.

Morris-Yates, A., Andrews, G., Howie, P., & Henderson, S. (1990). Twins: A test of the equal environments assumption. Acta Psychiatrica Scandinavica, 81, 322-326.

Mowrer, E. R. (1954). Some factors in the affectional adjustment of twins. American Sociological Review, 19, 468–471.

Nichols, R. C., & Bilbro, W. C. (1966). The diagnosis of twin zygosity. Acta Genetica et Statistica Medica, 16, 265-275.

Shields, J. (1954). Personality differences and neurotic traits in normal twin schoolchildren. Eugenics Review, 45, 213-246.

Torgersen, S. (1979). The determination of zygosity by means of a mailed questionnaire. Acta Geneticae Medicae et Gemellologiae, 28, 225-236.

von Bracken, H. (1934). Mutual intimacy in twins. Character and Personality, 2, 293-309.

Wilson, P. T. (1934). A study of twins with special reference to heredity as a factor determining differences in environment. Human Biology, 6, 324-354.