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IN 1979, the US public was fascinated by news coverage of the 鈥淛im twins鈥�, a pair of identical twin brothers who were adopted at birth by different families, only to find each other at the age of 39.
The coincidence of their matching first names wasn鈥檛 their only similarity. They weren鈥檛 mirror duplicates of each other, in looks or temperament, but both worked in law enforcement and their hobby was carpentry. Both owned Chevrolets and took vacations at the same beach in Florida. Even more improbably, they had both married women named Linda only to divorce them and later marry a Betty.
The Jim twins helped spark an important and long-running study in the field of genetics, the . This compared 137 pairs of identical and non-identical twins who grew up separated, as well as later comparing them with twins raised in the same family. It was among the first to show that about half of the variation in people鈥檚 personality is down to heredity, contradicting the prevailing blank slate ideas of the time.
The age-old nature versus nurture debate often gets a bad press, thanks to a long history of oversimplification and distortion to support dubious political ideologies. It is a shame, as this field often sheds light on some of the most interesting questions about what makes us who we are.
In Unique: The new science of human individuality, David Linden, a neuroscientist at Johns Hopkins University in Baltimore, Maryland, tours the latest research on the great diversity of human behaviour and physiology. He looks at how we are shaped by genes, upbringing and chance, covering everything from sex and sexuality to how we sleep and how we sense the world around us.
鈥淎side from extreme cases, the measurable effects of parenting on personality are often under 10 per cent鈥�
One thing that might seem fixed is our ability to discriminate between odours, he says, yet it is more malleable than we think. In high-income nations, smell is often seen as the poor relation of other senses such as vision and hearing, but people in hunter-gatherer communities, like the Maniq of Thailand, tend to be better at identifying odours in tests, presumably because they grow up needing to track down their food.
This difference in people鈥檚 ability to use smell shapes their language. In English, for example, there are few words to describe smells that aren鈥檛 related to their source. We might say something smells smoky or fruity, but there are no abstract descriptors. In the Maniq language, however, there are 15 abstract words for odours.
An often-overlooked influence on our lives is the sheer randomness of embryonic development. When those twin studies showed that about half of variation in personality is genetic, it was long assumed that the rest came from how we are reared. But other kinds of studies have found that, aside from in extreme cases such as child abuse, the measurable effects of parenting on things like IQ and personality are small 鈥� often under 10 per cent.
Geneticists such as Robert Plomin at King鈥檚 College London argue that much of the remaining variation is the result of chance events as our brains develop in the uterus. As Linden puts it: 鈥淭he wiring diagram of the human brain is so enormous and complicated that it cannot be specified exactly in the sequence of an individual鈥檚 DNA. Subtle random changes in the position or movement of cells within the developing nervous system can cascade through time to produce important differences in neural wiring and function.鈥�
There is so much that is still unclear about brain development that no one can say how many of the Jim twin coincidences were due to flukes of fate or shared DNA. We are only at the start of our journey to understand the human brain, but Linden鈥檚 book offers some very welcome signposts.
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