Babel No More: The Search for the World's Most Extraordinary Language Learners. By Michael Erard. Free Press; 306 pages; $25.99. Buy from Amazon.com

CARDINAL MEZZOFANTI of Bologna was a secular saint. Though he never performed the kind of miracle needed to be officially canonised, his power was close to unearthly. Mezzofanti was said to speak 72 languages. Or 50. Or to have fully mastered 30. No one was certain of the true figure, but it was a lot. Visitors flocked from all corners of Europe to test him and came away stunned. He could switch between languages with ease. Two condemned prisoners were due to be executed, but no one knew their language to hear their confession. Mezzofanti learned it in a night, heard their sins the next morning and saved them from hell.

Or so the legend goes. In “Babel No More”, Michael Erard has written the first serious book about the people who master vast numbers of languages—or claim to. A journalist with some linguistics training, Mr Erard is not a hyperpolyglot himself (he speaks some Spanish and Chinese), but he approaches his topic with both wonder and a healthy dash of scepticism.

Mezzofanti, for example, was a high-ranking clergyman born in 1774. In most of his interactions, he would have been the one to pick the topic of conversation, and he could rely on the same formulae he had used many times. He lived in an age when “knowing” a language more often meant reading and translating rather than speaking fluently with natives. Nonetheless, Mezzofanti clearly had speaking talent; his English accent was so good as to be almost too correct, an Irish observer noted.

To find out whether anyone could really learn so many languages, Mr Erard set out to find modern Mezzofantis. The people he meets are certainly interesting. One man with a mental age of nine has a vast memory for foreign words and the use of grammatical endings, but he cannot seem to break free of English word-order. Ken Hale, who was a linguist at the Massachusetts Institute of Technology and died in 2001, was said to have learned 50 languages, including notoriously difficult Finnish while on a flight to Helsinki. Professional linguists still swear by his talent. But he insisted he spoke only three (English, Spanish and Warlpiri—from Australia's Northern Territory) and could merely “talk in” others.

Mr Erard says that true hyperpolyglottery begins at about 11 languages, and that while legends abound, tried and tested exemplars are few. Ziad Fazah, raised in Lebanon and now living in Brazil, once held the Guinness world record for 58 languages. But when surprised on a Chilean television show by native speakers, he utterly flubbed questions in Finnish, Mandarin, Farsi and Russian (including “What day is it today?” in Russian), a failure that lives in infamy on YouTube. Perhaps he was a fraud; perhaps he simply had a miserable day. Hyperpolyglots must warm up or “prime” their weaker languages, with a few hours' or days' practice, to use them comfortably. Switching quickly between more than around six or seven is near-impossible even for the most gifted.

Does that mean they don't really know them? Is instant availability of native-like competence the only standard for “knowing” a language? How should partly knowing a tongue be tallied? What if you can only read in it? Mr Erard repeatedly peppers his text with such questions, feeling his way through his story as a thoughtful observer, rather than banging about like an academic with a theory to defend or a pitchman with a technique to sell.

Hyperpolyglots are more likely to be introverted than extroverted, which may come as a surprise to some. Hale's son always said that, in his father's case, languages were a cloak for a shy man. Another, Alexander Arguelles, has learned dozens of languages only to read them, saying “It's rare that you have an interesting conversation in English. Why do I think it would be any better in another language?” Emil Krebs, an early-20th-century German diplomat who was also credited with knowing dozens of languages, was boorish in all of them. He once refused to speak to his wife for several months because she told him to put on a winter coat.

Different hypotheses may explain part of the language-learner's gift. Some hyperpolyglots seem near-autistic. In support, Mr Erard points to the theory of Simon Baron-Cohen, of Cambridge University, that autists have an “extreme male brain” that seeks to master systems. Another hypothesis is the “Geschwind-Galaburda” cluster of traits. Supposedly resulting from abnormal antenatal exposure to hormones, this cluster includes maleness, homosexuality, left-handedness, poor visual-spatial skills, immune disorders, and perhaps also language-learning talent. Brain areas are also keyed to certain skills. The left Heschl's gyrus is bigger than average in professional phoneticians. People who learn new vocabulary quickly show more activity in the hippocampus. Krebs's brain, preserved in slices at a laboratory in Düsseldorf, shows various unusual features.

The discovery of the FOXP2 brain gene, a mutation of which can cause language loss, was met with considerable excitement when it was announced over a decade ago. But the reality is that many parts of the brain work together to produce speech and no single gene, region of the brain or theory can explain successful language-learning. In the end Mr Erard is happy simply to meet interesting characters, tell fascinating tales and round up the research without trying to judge which is the best work.

At the end of his story, however, he finds a surprise in Mezzofanti's archive: flashcards. Stacks of them, in Georgian, Hungarian, Arabic, Algonquin and nine other tongues. The world's most celebrated hyperpolyglot relied on the same tools given to first-year language-learners today. The conclusion? Hyperpolyglots may begin with talent, but they aren't geniuses. They simply enjoy tasks that are drudgery to normal people. The talent and enjoyment drive a virtuous cycle that pushes them to feats others simply shake their heads at, admiration mixed with no small amount of incomprehension.