For some decades, we heard claims from studies of identical twins (formed when one fertilized egg splits) that everything from exam results to homosexuality might hinge on genetics. Therefore, any similarity in later choices or behavior might be due to genetic factors (read “predetermined” or “inevitable” here). How has that assumption held up, especially in the age of genome mapping?

Identical twins comprise roughly 1 in every 250 births. Studies of twins who were separated at birth, have been especially prized because the twins were assumed to grow up in different environments. Thus any significant similarities pointed to genetic influences.

Several problems emerged though. For one thing, what about the assumption that separation at birth means that twins experience different environments? Children may be born into practically any environment but they are generally adopted into middle-of-the-road ones. Twins separated at birth may also be aware each other’s existence; they may even know each other. Plus, the psychological tendency we have when encountering twins is to notice similarities more readily than differences so similarities, rather than differences, tend to be socially reinforced. But the critics who raised these issues were typically ignored in the rush to see genetics behind every similarity.

Meanwhile, there were two bigger problems.

1.Identical twins diverge genetically as they age.

Genome mapping has changed the picture a good deal. The fact that twins diverge as they age was reported in Nature back in 2005. A 2021 study found that about 15% of identical twins vary from each other significantly in genetics. In any event, a 2022 UC Berkeley open-access study found that “age plays a more important role than genetics in determining which genes in our bodies are turned on or off, influencing our susceptibility to disease”:

In other words, while our individual genetic makeup can help predict gene expression when we are younger, it is less useful in predicting which genes are ramped up or down when we’re older — in this study, older than 55 years. Identical twins, for example, have the same set of genes, but as they age, their gene expression profiles diverge, meaning that twins can age much differently from each other. – Yamamoto, R., Chung, R., Vazquez, J.M. et al. Tissue-specific impacts of aging and genetics on gene expression patterns in humans. Nat Commun 13, 5803 (2022). https://doi.org/10.1038/s41467-022-33509-0

So, it turns out, even if we didn’t start out that way, we all end up being unique.

2.The effort to prove that Genes Rule! involved some avoidable lapses

In an article published at Aeon last month, science writer Gavin Evans, author of Skin Deep: Journeys in the Divisive Science of Race (OneWorld 2019) who follows twin studies, looks back on an era when the haste to establish genetic explanations for human behavior involved throwing ethics aside.

In the mid-twentieth century, Sir Cyril Burt (1883–1971), a British psychologist and eugenics enthusiast, claimed to have participated with colleagues in studies on separated identical twins that established the importance of heredity. But then, as Britannica decorously puts the matter,

After Burt’s death, striking anomalies in some of his test data led some scientists to reexamine his statistical methods. They concluded that Burt manipulated and probably falsified those IQ test results that most convincingly supported his theories on transmitted intelligence and social class. The debate over his conduct continued, but all sides agreed that his later research was at least highly flawed, and many accepted that he fabricated some data. However, the soundness of his earlier work justified his reputation as the foremost pioneer of educational psychology in Britain. – Britannica, The Editors of Encyclopaedia. “Sir Cyril Burt”. Encyclopedia Britannica, 27 Feb. 2023. Accessed 8 July 2023.

Evans tells the same story a bit more colorfully:

Shortly after he died in 1971, Burt’s records and notes were all burnt, after which his reputation imploded. Two of his researchers, whose names appeared as co-authors on his papers, could not be traced (when asked about them, Burt had said they’d both ‘emigrated’ – but he didn’t know where) and a third he clearly invented. In The Science and Politics of IQ (1974), the American psychologist Leon Kamin noted that in 1955, when Burt claimed to have tested 21 separated identical twins, he put the correlation between their IQs at 0.771, yet in the 1960s, when his twins cohort numbered 53, he gave the identical three-decimal figure, which Kamin said had a statistically minuscule chance of occurring. Some circumstantial details that Burt claimed to have found among his twins also raised eyebrows: of a pair born to a wealthy mother and then adopted, he claimed one was raised in splendour on a Scottish country estate, and the other was left to a shepherd (like Perdita in The Winter’s Tale). The killer blow was delivered by his approved biographer, Leslie Hearnshaw, a one-time Burt enthusiast who in 1979 concluded that all of Burt’s twin studies were invented. – Gavin Evans, “The myth of mirrored twins,”. Aeon, June 27, 2023.

Evans found similar, though less dramatic, problems with Thomas J. Bouchard ’s research at Minnesota Center for Twin and Family Research:

… his methods and conclusions did not impress other researchers. One problem was self-selection. His identical twins had known each other for an average of nearly two years before contacting him; some had known each other as young children; and it seems likely that those who were most alike were most likely to contact him. Kamin, the professor who rumbled Burt’s fraudulent studies, and his colleague said there was pressure on the twins to come up with cute stories, and that Bouchard’s studies had ‘a number of serious problems in the design, reporting, and analyses’. – “The myth of mirrored twins,” Aeon, June 27, 2023.

What is it about twins?

Is there something existential about twins that draws (and maybe, skews) research? Evans thinks so:

Much of the magic evaporates when we lift the lid on the sensational tales of parallel lives. What emerges in place of this seductive mirror myth of the hidden double are more mundane tales of everyday difference, revealing the unique selfhood that is part of the inheritance of all people – including those with genetic doppelgängers. – “The myth of mirrored twins,”. Aeon, June 27, 2023.

Yes, it seems we are all condemned to just be ourselves, even if we are one of a set of twins.

Note 1: Josef Mengele’s monstrous concentration camp experiments with twins in World War II are a much more dreadful story along these lines.

Note 2: According to Gavin Evans, the genetic influence on homosexuality is “minor with gay men, and even smaller with lesbian women.”

You may also wish to read: How can the two-headed tortoise have different personalities? Many would be surprised to learn that either head had any personality, and yet… Researchers have found that tortoises are really slow but smarter than we thought. However, their neurology differs from ours.