Consider Jack and Oskar
- Born Together – Reared Apart: The Landmark Minnesota Twin Study by Nancy Segal
Harvard, 410 pp, £39.95, June 2012, ISBN 978 0 674 05546 9
In a tongue-in-cheek editorial in the February 1927 issue of the Journal of Educational Research, the psychologist Guy Whipple announced that ‘the age-old perplexity of heredity has been banished; the old riddle of nature versus nurture has been solved.’ For the previous half-century, psychologists, geneticists, pedagogists and eugenicists had been trying to determine how personality attributes such as intelligence, manual skill and temperament were passed from parents to their offspring. Now, Whipple deadpanned, a rising movement in psychology had invalidated this whole line of research, insisting instead that ‘there are no inherited traits, characters, talents; that every normal person is born with the capacity to learn any behaviour that man has ever known, and that a body organisation [i.e. psychological constitution] can be built up in the first five years of life which makes it impossible for the person to kill or steal.’ This may be a tall order to place on teachers, Whipple continued, but too bad: anyone who wanted to keep up with the latest scientific thinking must embrace ‘nurture’ and ‘kiss Nature goodbye’.
For the next half-century, American psychologists did, overwhelmingly, come to base their work on the principle that it is environment – society, culture, upbringing, the random events that make up day to day life – and not heredity which has the greatest sway over the differences between individuals. Nancy Segal’s Born Together – Reared Apart tells the story of one effort to return to hereditarian accounts: the Minnesota Study of Twins Reared Apart, or Mistra, which between 1979 and 1999 examined 137 pairs of identical and fraternal (non-identical) twins separated in early childhood and raised in different households, assessing such traits as intelligence, conservatism, personal dynamism, creativity, religiousness and sexuality. After a series of more than four thousand tests, as well as observations and interviews with the twin pairs, Mistra scientists proposed that a huge number of personality traits previously thought to be influenced by environment and upbringing – such things as career choice, reading habits, food preferences, when we have sex and whom we tell about it – were, in fact, driven by genes. ‘Behaviour[al] geneticists,’ Segal declares, ‘have shown that virtually all measured traits display genetic variation.’
Segal spent three years first as a postdoctoral fellow under the study’s director, Thomas Bouchard, and then as the assistant director of the Minnesota Center for Twin and Adoption Research. Her book answers critics of Mistra who argue, as Joseph Jay did in the American Journal of Psychology in 2001, that ‘the studies of separated twins contain serious flaws, and the authors’ conclusions are questionable,’ and attempts to demonstrate the power of genetics to shed new light on human behaviour. Yet what emerges clearly in the course of the book is less a sense that nature has been vindicated, or that a new age of insight has dawned governed by behavioural genetics, but rather a feeling that the big questions about nature and nurture have gone begging.
In 1876, Francis Galton published ‘The History of Twins, as a Criterion of the Relative Powers of Nature and Nurture’. Fascinated by ‘mental heredity’ but unable reliably to distinguish the attributes his subjects were born with from ‘those that were imposed by the circumstances of their after-lives’, Galton hit on a novel idea: assemble a sample set of identical twins, ask them about their life histories, physical development, moods and intellectual aptitudes, then discern which attributes seemed most prone to environmental influence and which to heritability. His survey of 35 twin pairs showed – quite contrary to his expectations – that the dispositions of adult twins tended to show remarkable similarities, sometimes in spite of ‘very different conditions of life’. This suggested to Galton that mental traits were much more heritable than he had anticipated, and also that the twin method was a viable research tool.