In real life, such reports have come trailing in the wake of data gathered by scientists. At the University of Minnesota, Thomas Bouchard has studied about 250 twins who were raised apart. These separations are seized upon as real-world experiments - usually accidental, but not always - in which the genes are the constant and the environment variable. The outcomes are perceived as reports on the limits of life's possibilities; of whom, in different circumstances, a person might have been.
The message from Minnesota is that at a myriad points of comparison, there are no alternatives. The uncanniness is in the details. One of a pair flew into Minneapolis from San Diego, the other from Munich. Meeting for the first time in 35 years, they shook hands at the airport wearing blue shirts with epaulettes, glasses with rectangular wire frames, and clipped moustaches. They turned out to share a habit of wrapping elastic bands round their wrists, they flushed toilets before use as well as after, and found it amusing to sneeze loudly in a silent group of people.
Anecdotes like these have captured the public imagination, and turned several pairs of reunited twins into legend. The moral of these tales is much the same as that of Cavafy's poem "The City": there is no point in going to another country, to another shore, for you will remain essentially the same person. Raised in some jungle, either of those twins would have twisted vines round their wrists instead of elastic bands.
For human essences, though, these have peculiarly inconsequential effects. Jack and Oskar may have fidgeted in the same way at their desks and shared a somewhat bumptious sense of humour, but their lives were guided by grand narratives that were anathema to each other. They were born in Trinidad in 1933; after their parents' divorce, Jack remained there with his Jewish father, while Oskar was sent to the household of his maternal grandmother, who was German, Catholic and Nazi. Little love was lost between the twins over the years, and having quirks in common was apparently not a sufficient basis for a warm relationship when they finally spent time together. While their temperaments were similar in many ways, their identities were profoundly different.
Yet the details speak louder than the bigger picture. Reflecting on the fantasy of other lives, Wright at first sounded like an author who had just succeeded in prising open the lid of imagination. With his book complete, however, his tone has flattened.
He accepts it all, from coefficients of heritability to tales of the uncanny. The subtitle, "Genes, Environment and the Mystery of Human Identity", has a denouement: it's all in the genes. Our environment is largely irrelevant except in extreme circumstances, and so the mystery of human identity evaporates.
Accepting genetic determinism seems to have a woeful effect on Wright's critical faculties, and his moral reflexes. His face is straight as he describes how one of Bouchard's colleagues, David Lykken, tried to test whether twins sent each other telepathic messages. It stays straight as he outlines Lykken's ideas about licensing parenthood. Under this modest proposal, women without licences for pregnancy would be confined to maternity homes, and their babies taken away for adoption immediately after birth.
Here and throughout the book, Wright adopts a descriptive posture. He acknowledges criticisms of twin research, such as the observation that film deals give a financial incentive to twins' self-mythologising tendencies, without really engaging with them.
This approach allows him to present the current climate of opinion as a fait accompli, and by implication an expression of fundamental truths. He links the triumph of western conservatism to the point made by twin studies: "we don't become. We are."
Behaviour genetics, with twin studies at its heart, is the truly dismal science of today. The metaphor that Wright has devised to convey its message fits only too well. "Let us say you are as alike as two different McDonald's restaurants in separate cities - the same architecture, the same inner decor, the same employee uniforms, the same menu. The environment outside may be Des Moines or Dallas, but the structure of the restaurant is the same. The experience of being in either place is hauntingly unvaried."
Though evolutionary psychology also thrives in the prevailing climate, levels of heritability are not its chief preoccupation. It prefers to talk about human universals rather than about the universal limits to human possibilities, although this choice is influenced by the desire for a positive spin.
That doesn't wash with Steven Rose, whose new book Lifelines is the polar opposite of Twins. Arguing that the nature-nurture dichotomy is sterile, he notes that we are all interactionists now. But for him this means that the interactions are too complex to resolve. Projects such as the investigation of human nature are impossible in principle.
This makes Rose's line harder than that of the most prominent scourge of determinism, Stephen Jay Gould. Last June, a two-part essay by Gould appeared in the New York Review of Books. Although it was another broadside in the long exchange of fire between Gould and some of Darwinism's most notable theorists, it made two fundamental concessions.
"Humans are animals and the mind evolved; therefore, all curious people must support the quest for an evolutionary psychology," wrote Gould, although made it clear that he considers existing evolutionary psychology to be no better than "cocktail party conversation". Yet he acknowledged its most powerful idea: that males and females have different reproductive interests because they incur different costs.
Gould is a palaeontologist, contemplating the processes of evolution; Rose is a neuroscientist, examining how living systems work. While Gould operates like a prolix Clint Eastwood, riding solo into Darwin City's main street, Rose employs the tactics of the political activist to build alliances.
Rose draws more support from radical critics of science than from evolutionary theorists. His own science emphasises the organism, and its development, rather than the gene. In this view of life, challenges from behaviour genetics and modern Darwinism simply don't figure. Twin studies, for example, are dismissed in a page or two.
Rose is not really very interested in evolution, but he is greatly offended by some of its current manifestations. Like Gould, he uses the prefix "ultra" to condemn modern Darwinism, but his more telling pejorative epithet is "vulgar".Though kin to the term "vulgar Marxism", Rose uses it in the way it is applied to the Duchess of York. Unlike Wright, Rose leaves the reader in no doubt about his personal sensibilities. Modern Darwinists and their hereditarian cousins, he feels, have deplorable taste.
Rose may be right to emphasise how much we could learn from studying organisms in the round. However deeply we explore the origins of individuals, though, that still leaves the question of the origins of species. In a review in Nature last April, Rose suggested a moratorium on books with "evolution" in the title. But the mind still evolved, as Gould has pointed out, and wanting to understand how is only human, after all. As for heritability, it is too important to be left to the kind of person who would lock up unlicensed mothers.