IN 1875, three decades before genes officially arrived on the scientific scene, : 鈥淭wins have a special claim on our attention鈥� their history affords means of distinguishing between the effects of tendencies received at birth, and those that were imposed by the special circumstances of their after lives.鈥� With today鈥檚 flood of information from genetics, and with results from many studies of monozygotic (identical) twins raised together and apart, the promise implicit in Galton鈥檚 observation should be close to fulfilment. Are we on the brink of knowing conclusively which of nature and nurture is dominant in making us what we are?
Whereas we know that some things (eye colour, Huntington鈥檚 disease) are definitely genetic, and others (the language we speak or the religion we adhere to) are environmental, we wish to know the degree of heritability of such traits as intelligence, schizophrenia and aggression. To date, the evidence from studies of separately raised monozygotic twins favours the nature side of the argument, which in turn suggests that we have to accept a significant degree of biological determinism in our picture of humanity.
The main reason for resistance to this is, of course, the importance we attach to individual autonomy, moral choice and responsibility, and the possibility of making ourselves better people through education and experience. This has led some to challenge key concepts in behavioural genetics, including 鈥渢rait鈥�, 鈥渋ntelligence鈥� and 鈥渁ggression鈥�, along with the assumption that twins brought up in the same house really do share the same environment.
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Another response has been to argue that nature and nurture operate jointly to produce the wide palette of traits that constitutes an individual. This is plausible: environmental cues switch on genes in a number of cases (diet can diminish or exacerbate the symptoms of certain inherited disorders), and similar interactions might be found for many environment-gene relationships. In this vein, the New Zealand IQ expert James Flynn argues that , suggesting that twin studies might overestimate the role of genes in this case.
But there is one source of evidence that has been under-used. This is the case of certain genetic syndromes which make unrelated individuals seem like twins, or at least exhibit extremely similar repertoires of physical and mental traits. Such cases add weight to the nature case, and merit study accordingly.
鈥淣ature is producing unrelated 鈥榯wins鈥� with similar physical and mental traits鈥�
Examples are syndromes that result from small deletions or additions of DNA in chromosomes. Some are too rare or too recently observed to have acquired names, and are known by their DNA coordinates. The six children so far identified with deletions of DNA in chromosome 16 () all look alike, with flat faces, deep-set eyes and small chins, and experience similar developmental and behavioural difficulties. The 370 individuals worldwide diagnosed with deletions in chromosome 22 () tend to share the syndrome鈥檚 characteristic flaky toenails, puffed cheeks and poor muscle tone, and manifest symptoms similar to those of autism.
In these and similar cases nature is producing otherwise unrelated 鈥渢wins鈥� 鈥� with a specific genetic difference from the norm. They all need attentive care and therapy, so might it be that among the gifts they could bring is added insight into the difference made by environmental inputs targeted to their specific genetic syndrome? For here we have clear cases in which the relative outcomes of a specific genetic endowment and responses to it from the environment can be significantly compared.
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