1. Newman et al. (1937). Twins: A Study of Heredity and Environment. Chicago: University of Chicago Press.
This book presents the first comprehensive psychological and physical study of 19 sets of MZA twin pairs. Some people may not realize that this study also includes comparative data on 50 MZT and 50 DZT twin pairs. The work, which took place at the University of Chicago, was a collaboration among a biologist (Newman), educational psychologist (Freeman), and statistician (Holzinger). The chapters include a literature review, critique of methods, causes of identical twin differences, and statistical analyses of twins reared apart and together. One of the best features of this book is its compendium of life history events, behavioral results, physical findings, and photographs for each of the 19 reared-apart pairs. The coupling of quantitative and qualitative data is rare today given participant confidentiality issues, thus increasing the value of this material.
A follow-up paper to the 1937 study by Gardner and Newman (1940) is well- worth reading. The investigators report findings for a newly discovered MZA female pair, while regretting that one member of a new MZA male pair refused research participation. As I noted in BT-RA (Segal, 2012), Newman regretted losing this pair because of their marked personality differences.
2. Burlingham (1952). Twins: A Study of Three Pairs of Identical Twins With 30 Charts. London: Imago Publishing.
Burlingham's Twins is an extraordinary record of the early behaviors of three young MZ twin pairs, Bessie and Jessie, Bert and Bill, and Mary and Madge. The children were variously observed from birth to age 5 years at the Hampstead Nurseries, a residential facility financed by the Foster Parents’ Plan for War Children, Inc., in New York. The study was intended to identify the ‘environmental and innate conditions which account for the differences between their development and that of ordinary children’ (p. x). Other intriguing topics include the fantasy of having a twin, twins’ reactions to separation and parent–twin relations. My favorite chapter is ‘The Beginning of the Twin Relationship’, an informative look at co-twins’ first awareness of one other — a topic rarely considered. Detailed charts tracking each child's behavioral and physical development are provided.
3. Shields (1962). Monozygotic Twins: Brought Up Apart and Together. London: Oxford University Press.
Shield's study presents findings from 44 MZA twin pairs and some additional data on 11 DZA twin pairs. Like the Newman et al. (1937) study, Shields provided a terrific overview of the field, statistical findings on physical variables, intelligence and personality, and appended informative case material for each twin pair with names altered to protect the twins’ identity. Shields was a great thinker and a great observer of behavior. One of my favorite passages is his take on the social attraction between MZA co-twins: ‘The youngest pair, S f 1, who became attracted to one another without realizing they were twins, gave rise to the reflection that there might be a biological basis for the frequently observed close attachment of monozygotic twins. Cattle twins, when reunited after segregation, can pick out one another from others in the herd . . .’ (p. 51). This reasoning anticipated issues currently being addressed by evolutionary-based researchers.
4. Rosenthal (1963). The Genain Quadruplets. New York: Basic Books.
I read this book in 1974 when I learned I would be spending the summer at NIMH in Bethesda, Maryland, coding follow-up data on the Genain Quadruplets. The four MZ sisters (whose falsified first names follow the letters of the institution where they were studied: Nora, Iris, Myra, and Hester), were concordant for schizophrenia, albeit with different symptoms that varied in severity. The book was published at a time when the roots of schizophrenia were linked to parental upbringing and dysfunctional family patterns. However, Rosenthal and his colleagues underlined contributions from both genetic and environmental sources. They explained the disorder with reference to diathesis–stress theory, asserting that behavioral disorders are outcomes of both genetic and environmental risk factors working in concert.
5. Juel-Nielsen (1965/1980). Individual and Environment: Monozygotic twins Reared Apart. New York: International Universities Press.
Juel-Nielsen conducted the only reared-apart twin study that was population based. His interest in reared-apart twins began when an outpatient at the State Hospital in Risskov, Denmark mentioned that she had been separated from her twin at 3 weeks of age. Several weeks later, Juel-Nielsen was approached by a journalist researching a similar case. Given the availability of population registries in Denmark, he located 10 additional MZA twin pairs born between 1870 and 1910. This study, which compared the twins’ rearing homes, general intelligence, personality traits, health-related measures, and psychiatric symptoms, was first published in 1965. A 1980 edition, with an introduction by Irving Gottesman, included 20-year follow-up data. The twins’ life histories (with altered names) complement the quantitative results. A 1981 interview with Juel-Nielsen, conducted at the University of Minnesota by Tom Bouchard, provides a rare inside view of his findings, his thinking about the findings and their impact on the field; see Segal (2012).
6. Koch (1966). Twins and Twin Relations. Chicago: University of Chicago Press.
Koch's meticulously done study compares similarities and differences in cognitive measures, personality variables, and physical characteristics among 5- and 6- year-old twin pairs, organized by zygosity and sex. The social relationship qualities of each type of twin are also examined. In addition to its quantitative findings, the work includes qualitative accounts of many aspects of twinship. Koch's comparative descriptions of the psychological situations of the five types of twins (MZm, MZf, DZSSm, DZSSf, DZOS) remain among the best in the field.
7. Scheinfeld (1967). Twins and Supertwins. New York: J.B. Lippincott, Williams & Wilkins.
I believe this is the first twin research overview I read and I now own five copies of this book — the one I bought, the one I received from Gottesman, the two I received from Kay Cassill (former director of the Twin's Foundation in Providence, Rhode Island) — and the signed copy I received from Amram Scheinfeld in June 1977 when I met him and his wife Dorothy in New York City. I still love this book with its attention to psychological differences among twin types, significant twin-related issues (e.g., school separation, individuality), informative illustrations, twinning statistics, and list of ‘trailblazers’ in the twin research field. I consult this wonderful volume from time to time because it contains material that is not included in other sources. For example, it was here that I learned about the first known case of switched-at-birth twins in Fribourg, Switzerland, and the names of the physicians who documented the genetic relatedness of the separated twin boys. A book written by the twins’ mother is also worth reading; see below. Scheinfeld also discusses psychological and biological aspects of higher order multiples, and twins in myth, folklore, and literature.
8. Bulmer (1970). The Biology of Twinning in Man. London: Oxford University Press.
Bulmer's book on the biology of twinning has stood the test of time. Of course, more is known today about the genetics of twinning, biological variations among the different twin types, and changing population frequencies (e.g., Benin now delivers more twins than Nigeria; see Smits & Monden, 2011) than when the book appeared in 1970. Specifically, Benin's 2006 twinning rate was 27.9/1,000 births, with 53,595 live births in 10 years before the survey (1999–2008); Nigeria's 2008 twinning rate was 19/1,000 births with 83,003 live births (1996–2006). In addition, Bulmer's work was completed prior to the advent of assisted reproductive technologies that have dramatically altered twinning rates in Western countries. Still, Bulmer's book qualifies as a classic because of the material it presented at the time. It also presents one of the clearest and most complete discussions of polar body twinning in the literature.
9. Gottesman & Shields, (1972). Schizophrenia and Genetics: A Twin Study Vantage Point. New York: Academic Press.
‘Gottesman & Shields’ is a household term among twin researchers, the joining of two great names in a stunning collaboration that put forward a multifactorial polygenic threshold theory. This comprehensive twin study of schizophrenia, published in 1972, identified 75 twin pairs (24 MZ and 33 DZ) from the Maudsley Twin Register maintained by the Psychiatric Genetics Research Unit of the Medical Research Council, in London. The 57 pairs in the study included 62 probands who had been consecutively admitted into the hospital. The project broke new methodological ground in its quest to determine the effects of diagnosis on concordance rates. Patient summaries (that did not include zygosity or diagnosis) were distributed to six senior judges from the UK, USA, and Japan. Following categorization of the judges’ classifications and elimination of two MZ twin pairs that did not include a schizophrenic proband, the proband-wise concordance rates were: 15/26 or 58% (MZ) and 4/34 or 12% (DZ), consistent with strong genetic effects. Other valuable features of this volume include overviews of genetic family studies, case studies of the participating pairs (personally interviewed) and a comparative evaluation of twin studies of schizophrenia. A foreword by Eliot Slater and an in-depth ‘critical afterword’ by Paul Meehl are enlightening.
10. Loehlin & Nichols, (1976). Heredity, Environment, and Personality: A Study of 850 Sets of Twins. Austin, TX: University of Texas Press.
Loehlin and Nichols's work is monumental, but not only because of its sample size (N = 850 twin pairs) and its wide range of topics (intelligence, personality, social relatedness, rearing factors). This study addressed (and I believe generally resolved) issues that are still being debated today. For example, the equal environments assumption (the concept that MZ and DZ twins are treated alike with reference to a given measure) was examined in analyses of similarity/dissimilarity of parental treatment and child behaviors. The investigators showed that whether or not parents treated MZ twins alike was unrelated to their developmental outcomes. Researchers will continue to find new bits of information and analyses upon subsequent readings. Whenever I research a new subject, I usually consult Loehlin and Nichols's study to see if they had considered it previously. They often had.
Other works deserving of honorable mention are those by Newman (1917) on the biology of twinning, Newman (1940) on human twins, triplets and more, Lindeman (1969) on a pair of MZ co-twins who met at age 25, and Mittler (1971) on the findings and implications of twin research. The series of studies conducted on the identical Dionne quintuplets is well known (Blatz et al., 1937), but conducted under circumstances insensitive to the family's well-being. A little known book highly deserving of mention is He Was Not My Son, by Madeleine Joye (1954), the mother of switched-at-birth MZ male twins from Switzerland. Joye provided a compelling and heart-breaking account of her love for her ‘fraternal twins’, the realization that the boy she favored was not really hers, and the sad events following the return of each child to his biological family.
A paper on the top 10 classic papers in twin research is being planned. Suggestions are welcome; a copy of the manuscript should accompany all suggestions.