Author: Lawrence Wright


Twinning is undoubtedly one of the most enigmatic phenomena of reproduction, and this book addresses both the scientific and philosophical perspectives of this curious paradox. Admitting that ‘we don’t know who twins are or how twins happen’, the author nevertheless views twinning as a window for understanding ‘how we are the way we are’. Whilst the book highlights the ambiguities and riddles that shroud twins, depicting them as ‘an unsettling presence’ who ‘undermine our sense of individual uniqueness‘, it managed to extract meanings from the remarkable similarities and differences that exist between twins. Expectedly, the genetic underpinnings of twinning was central to the book, and the author extensively reviewed the role twin studies have played in determining the heritability of human traits. The remit of the book was however much broader as it also delved into the social, historical, and cultural dimensions of the subject, and this is reflected in such diverse topics as religion, worship, chastity, birth control, ostracism, and murder (pages 86, 6, 122, 25, 57 and 65).

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The biological processes that govern multiple pregnancies lie at the heart of the book, and the author’s account highlighted the complexities and uncertainties that characterise our knowledge of twinning. The book discussed many established facts about twins, and the development of identical or monozygotic twins from the fertilisation of one egg stands out as one of the most straightforward. On the other hand, the formation of fraternal or dizygotic twins is a less distinct process as it may result either from the fertilisation of two eggs, or from the fertilisation of one egg that had split before fertilisation. The complexity of twinning becomes more evident as pregnancy progresses, and the book reflected this when it pointed out that identical twins may either have the same or separate placentas and amniotic sacs, and this depends on when the zygote splits after conception. The author also explored the effect that the very late splitting of the zygote has on the development of conjoined twins. Perhaps in keeping with the complexity of its process, twin pregnancies are associated with diverse clinical consequences, and the book reviewed such complications as spontaneous abortion, premature birth, and birth defects such as spina bifida and heart defects. However, the most tragic risk of twin pregnancy the book highlighted is when late splitting twins die in-utero, ‘often strangulating in each other’s umbilical cords‘ (pages 11, 86-90, 102). CC-BY-4.0, CC BY 4.0, Link

The contentious history of twin research is another theme of the book which highlighted the conflicts that marred the legacy of twin studies (the bitterness of which the author said ‘has rarely been matched in the history of academic warfare‘). The book attributed the controversies of twin studies to ‘deeply held philosophical, political, or religious ideals’, and it illustrated this with the deplorable Nazi twin studies which were pioneered by Count Otmar von Verschuer and his protege Josef Mengele. The author described how, by their outrageous twin experiments, the Nazis sought to establish a link ‘between disease, racial types, and miscegenation‘, all in an attempt to ‘unlock the secrets of heredity and help create a master race‘. It is regretful that the Nazi twin studies ‘formed the foundation for much of modern medical research using twins’, the author stressing that they were typified by ‘wantonness and waste‘, and resulted in the death of almost 3,000 twins in Auschwitz. Another set of similarly influential but deceitful twin studies were those credited to Sir Cyril Burt – ‘Britain’s most honored and acclaimed psychologist’ of his day. In highlighting the ill effects of the devious history of twin research, the author contended that they were responsible for setting back genetic twin studies, and for allowing the unwarranted rise of the behaviourist movement pioneered by the psychologists John Watson and B. F. Skinner, the physician Benjamin Spock, and the educationist Charles Fries (pages 16-23 and 26-33).

By cliff1066 CC BY 2.0, Link
Even though the history of twins studies was replete with controversy, the author asserted that they nevertheless provided reliable data about the genetics of twinning, and about the heritability of medical disorders. For example, he observed that twin studies have demonstrated that there is ‘a strong genetic component to many forms of mental illness such as schizophrenia, phobias, and neuroses‘, and that medical disorders such as multiple sclerosis, autoimmune disorders, cancer, insomnia, and menstrual dysfunction, have significant heritable components. Even more remarkable is the knowledge garnered from twin studies which show that there are genetic elements to the risk of acquiring infections such as rubella and chickenpox, and of addiction to alcohol and cigarette smoking. The author also discussed the genetic mechanisms which may make identical twins to differ significantly from each other; for example he referred to how random X chromosome inactivation results in only one of the pair of identical twins inheriting an X-linked disease such as Duchenne muscular dystrophy. Similarly, the author noted that chimerism, the fusion of non-identical twins to form a single baby, may give rise to such unusual features as hermaphroditism, ambiguous genitalia, or the possession of different blood groups (pages 25, 64, 124, 94-95 and 105-109).

By Dennis Mathena, CC BY 3.0, Link

No account of twins is complete without the anecdotes of the uncanny similarities that have been documented between identical twins who have been raised by different families or in different environments. One such story the author narrated is of identical twins James Springer and James Lewis who, despite being raised apart from each other, had similar personalities and medical histories – they both married and divorced women named Linda; they remarried women named Betty; they gave the same names to their first borns and their dogs; they held the same jobs; and they both enjoyed the same activities. Even more unnerving similarities the author cited are the tendencies for twins to experience each other’s pain, and to share the same visual experiences when they are apart. The author tried to explain some of these extraordinary similarities, such as when he attributed the similar divorce histories of twins to their shared ‘personality characteristics that contribute to or detract from marital harmony‘ (pages 61-62, 45-50, 55-56 and 124-125).

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The author complemented his narrative with a unique exploration of the philosophical significance of twinning especially as it pertains to concepts such as freewill. For example, he considered how the ‘uncanny parallel existences‘ of separated identical twins pose a ‘most troubling question‘ about whether freewill even exists. Contending that twinning suggests that the whole concept of freewill is just a ‘wishful notion‘, he nevertheless cautioned against fatalism by emphasising that we still have the capacity to direct our natural traits ‘toward better or worse uses’. The book is also enhanced by the variety of stimulating concepts and enlightening facts it contains, and these included the interesting observation that left-handedness is frequent in twins; twins are more likely to be born to ‘tall, heavy women‘ and unmarried mothers; fingerprints of identical twins may manifest as mirror images of each other; identical twins may rarely have different genders when one twin has Down’s syndrome and missing a Y chromosome; and there is an extremely high rate of dizygotic twinning in the Yoruba tribe of Nigeria – ‘the world champions of twinning’ (pages 38, 154-157, 86, 99-100, 111, 113 and 119).


This most illuminating and enriching book is replete with facts and insights on a truly peculiar, but entirely natural, occurrence. With a detailed, even if occasionally disjointed and repetitious, narrative, the author covered the complete spectrum of concepts related to twinning. Going beyond the purely scientific, the author depicted the influence of twinning not just on healthcare but on society. The cultural and philosophical contexts the author provided are especially appealing, as are the striking anecdotes that illustrate the quirks of the subject.

Overall assessment

Twinning is a curious phenomenon, often steeped in myth and mystery, and the author of this book has conveyed these aspects of the topic as eloquently as he did its genetic and social dimensions. Just as it revealed the biological underpinnings of twinning, the book also demonstrated its cultural and philosophical implications. Of particular importance, the book reviewed the clinical risks and medical disorders associated with twin pregnancies. The book has treated the subject, which is relevant to healthcare, exhaustively, and I recommend it to all doctors.

Book details

Publisher, Place, Year: John Wiley and Sons, New York, 1997
Number of chapters: 10
Number of pages: 202
ISBN: 978-0-471-29644-7
Star rating: 4
Price: £11.99

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