Everyone has unique academic experiences as college students, doctoral candidates, medical students, professionals, and/or practitioners. There are certain research papers that stand out among others, based on our interests, but also because of their impact on a particular area of specialization. Such papers are variously classic, provocative and/or influential. Briefly summarized below is a selective sampling of articles that span over a century and which are those that I turn to often.
• Galton, F. (1876). The history of twins as a criterion of the relative powers of nature and nurture. Journal of the Anthropological Institute, 5, 391–406.
Galton eloquently set forth the logic of the classic twin design in a report of his findings from 35 twin pairs showing close similarity (presumably MZ twins) and 20 pairs showing marked dissimilarity (presumably DZ twins). These 55 pairs were drawn from a larger respondent pool of 80 sets, but these pairs provided the most detail. Note that the biological bases of twinning had not been worked out at that time. Also, note that Galton's recognition as ‘Father of the Twin Method’ has been a matter of some debate (Segal, 2017). It is in this 1876 paper that Galton stated that there is ‘no escape from the conclusion that nature prevails enormously over nurture’ (p. 404).
• Bouchard, T. J., Jr., Lykken, D. T., McGue, M., Segal, N. L., & Tellegen, A. (1990). Sources of human psychological differences: The Minnesota Study of Twins Reared Apart. Science, 250, 223–228.
IQ findings from MZA twin pairs in the Minnesota Study of Twins Reared Apart (MISTRA) were contrasted with those from three previous reared-apart twin studies on primary and secondary mental ability tests, respectively. The intra-class correlations proved exceedingly robust within and across studies: MISTRA (1990): (n pairs = 48, r i = 0.69; n pairs = 42, r i = 0.78), Newman et al. (1937): (n pairs = 19, r i = 0.68, n pairs = 19, r i = 0.74), Juel-Nielsen (1965): (n pairs = 12, r i = 0.64, n pairs = 12, r i = 0.73), and Shields (1962): (n pairs = 38, r i = 0.74, n pairs = 37, r i = 0.76). The MISTRA also reported results for a tertiary measure (n pairs = 43, r i = 0.78). Overall, approximately 70% of the IQ variance was attributable to genetic sources. Parental socioeconomic status, physical facilities in the home, and relevant family environment scales made negligible contributions to the MISTRA twins’ IQ scores and IQ resemblance.
• Gottesman, I. I., & Bertelsen, A. (1989). Confirming unexpressed genotypes for schizophrenia: Risks in the offspring of Fischer's Danish identical and fraternal discordant twins. Archives of General Psychiatry, 46, 867–872.
Gottesman and Bertelsen (1989) presented an elegant application of the twin-family study, also known as the MZ Half-Sibling Design, Nuclear Family Twin Design, Children-of-Twins Approach, and Twin-Pedigree Study. Using data gathered on MZ and DZ adult twins who were discordant for schizophrenia, the authors showed that the disease risk for children of affected and unaffected MZ co-twins was the same (16.8% and 17.1%, respectively). In contrast, the disease risk was elevated only among children born to affected DZ twins, in comparison with their unaffected co-twins (17.4% and 2.1% respectively). It was concluded that unexpressed disease liability may be transmitted to offspring.
• Tellegen, A., Lykken, D. T., Bouchard, T. J., Jr., Wilcox, K. J., Segal, N. L., & Rich, S. (1988). Personality similarity in twins reared apart and together. Journal of Personality and Social Psychology, 54, 1031–1039.
This paper presented the first four-group study of personality, including MZ and DZ twins reared-apart and together (MZA, DZA, MZT, DZT). MZA and DZA twin pairs came from the MISTRA, while MZT and DZT twins were enrolled in the Minnesota Twin Registry. The most provocative finding was that the MZA twins were as similar in personality as the MZT twins, as reflected by the median correlations across the 11 scales of the Multidimensional Personality Questionnaire (MPQ) (r is = 0.49 and 0.52, respectively). This result was wrongly interpreted by some as suggesting that parental rearing practices are inconsequential (Segal, 2012).
• Martin, N. G., Eaves, L. J., Heath, A. C., Jardine, R., Feingold, L. M., & Eysenck, H. J. (1986). Transmission of social attitudes. Proceedings of the National Academy of Sciences of the United States of America, 83, 4364–4368.
This paper reported the counterintuitive finding that individual differences in social attitudes can be partly explained by genetic factors. Twin participants included pairs from Australia and Great Britain. Social explanations, such as degree of contact between twins, could not account for the findings. This paper led to a number of replications by other research teams, among them the MISTRA, many of whom found comparable results.
• Merrill, J. P., Murray, J. E., Harrison, J. H., & Guild, W. R. (1956). Successful homotransplantation of the human kidney between identical twins. Journal of the American Medical Association, 160, 277–282.
This landmark case report documented the first successful kidney transplant between MZ twins. Given their genetic identity, MZ co-twins are ideal donors and recipients for one another, although infection and other factors may intervene (Segal, 2017). This procedure was, however, performed without immunosuppressive medication. The donor twin, Ronald Herrick, whose kidney was given to his twin brother at age 23, died 56 years later at age 79, from heart surgery complications. The recipient twin, Richard Herrick, lived for 8 years following the operation (Associated Press, 2010).
• Plomin, R., & Daniels, D. (1987). Why are children in the same family so different from one another? Behavioral and Brain Sciences, 10, 1–16.
This provocative and ground-breaking article underlined the finding that siblings growing up together in the same family differ considerably from one another, not just because of their different genes, but because of non-shared environmental influences. These influences appear to be the most important source of environmental effects on behavior, including cognitive skills. Such effects are shown most dramatically by adoptive siblings raised in the same home, whose correlations capture direct estimates of environmental influence. One might argue that the different ages and time of arrival in the home explain these siblings’ dissimilarity in general intelligence, but the same results have been reported for virtual twins, that is, same-age unrelated children raised together since early infancy (Segal et al., 2012).
• Lykken, D. T., & Tellegen, A. (1996). Happiness is a stochastic phenomenon. Psychological Science, 7, 186–189.
This paper used a complete four-group twin design (MZA, DZA, MZT, DZT) in an analysis of the subjective well-being scale of the Multidimensional Personality Questionnaire (MPQ). The MPQ had been completed twice by 79 MZT pairs and 48 DZT twin pairs, yielding a correlation of 0.50 for scores at the two different ages. More significantly, the MZT cross-twin, cross-time correlation of 0.40 was equal to 80% of the 0.50 retest correlation, whereas the same correlation for the DZT twin pairs was close to zero. This finding was replicated using the reared-part twins. It was concluded that genetic factors underlie each individual's characteristic happiness set point, that is, the general level of happiness that he or she experiences. Additionally, the well-being correlations did not differ between MZA (n = 75 pairs, r i = 0.52) and MZT twins (n = 647 pairs, r i = 0.44), demonstrating that shared environmental factors do not affect how happy we are or can become.
• Lykken, D. T., & Tellegen, T. (1993). Is human mating adventitious or the result of lawful choice? A twin study of mate selection. Journal of Personality and Social Psychology, 65, 56–68.
This thought-provoking piece challenged the view that our choice of mates follows lawful orderly processes. Instead, in a series of separate twin studies on assortative mating (twins and their spouses), spouse similarity (twins’ spouses), twins’ views of their co-twins’ choices and spouse evaluations of twins support the positions that mate selection reflects romantic infatuation, and that individuals’ characteristics have weak effects on partner choice. It is surprising that MZ twins do not choose similar mates, given their similarities in so many other areas of social behavior. Note that some recent research has found that MZ twins’ spouses and best friends are somewhat more similar to their co-twins’ friends and spouses than those of DZ twins in political orientation (Kandler et al., 2012), personality and attitudes (Rushton & Bons, 2005).
A personal anecdote may be informative. A number of years ago, I was on a British television program in which the members of four MZ twin pairs were individually introduced to four potential mating partners, themselves part of an MZ twin pair. Co-twins’ preferences were quite similar, suggesting that when the ‘same’ partner is available their choices coincide.
• McNamara, H. C., Kane, S. C., Craig, J. M., Short, R. V., & Umstad, M. P. (2016). A review of the mechanisms and evidence for typical and atypical twinning. American Journal of Obstetrics and Gynecology, 214, 172–191.
This article provides an excellent and timely overview of the many varieties of both typical MZ and DZ twin pairs. Variations with respect to placentation and chorionicity are described and supplemented by detailed diagrams created by the gifted medical illustrator Levent Efe. This combination of science and illustration informs even the most knowledgeable of twin research scientists, as does the debate surrounding traditional and new models of twinning. Other twinning varieties such as polar body twinning, fetus-in-fetu and fetus papyraceus are also covered. Appendices listing published reports of the rare monchorionic DZ twins, so called “mirror-image” twins, vanishing twins, superfetated twins and superfecundated twins are helpful to individuals interested in further researching these fascinating topics.
• Martin, N., Boomsma, D., & Machin, G. (1997). A twin-pronged attack on complex traits. Nature Genetics, 17, 387–392.
This paper critically examined twin studies in light of advances in molecular genetic techniques used for identifying quantitative trait loci. It was written over 20 years ago, but remains relevant today, given the rapidly accumulating analyses of twin data via molecular genetic procedures. Key themes are that classic twin studies must show genetic influence on traits of interest prior to undertaking more sophisticated examination; correct classification of twins as MZ or DZ is mandatory and easy to accomplish; and carefully conducted twin studies provide useful and valid data. A chart depicting various genetic and environmental factors associated with MZ co-twin differences is an especially informative feature of this publication.
• Turkheimer, E., Haley, A., Waldron, M., D'Onofrio, B., & Gottesman, I. I. (2003). Socioeconomic status modifies heritability of IQ in young children. Psychological Science, 14, 623–628.
Most twin studies include participants from middle-class families, but there is considerable interest in the extent to which findings vary across socioeconomic groups. This study compared IQ resemblance between 7-year-old MZ and DZ twin pairs from neighborhoods near or below the poverty level. In contrast with most extant findings, shared environmental effects explained 60% of the variance in general intelligence, with a negligible contribution from genetic factors. It appears that extreme settings can overwhelm individual differences in ability.
There are of course, many other studies I could have chosen, and not all readers of TRHG will agree with these choices. I look forward to learning the favorite papers of others working with twins and will consider listing them in a subsequent News, Views and Comments column.