Do you know how many words separate a middle-class child and a child from a deprived background by the age of five? About 32 million.
That is 32 million more words that the middle-class child has heard - one of the more startling figures in Maryanne Wolf's study of reading, Proust and the Squid, now available in paperback. The language gap between the deprived and privileged child represents, says Professor Wolf, "a disaster in the making". As her book shows, what we hear and what we read literally changes our brain.
"Every time you hear language, you develop pathways in the brain," she says. Reading and talking to their young children is the single most important thing parents can do to help them learn to read, she adds: "It truly enhances the child's ability to distinguish sounds within words and gives them an unconscious understanding of the structure of words and narrative." The human contact is vital: TV and audio tapes are not as effective.
As a teacher, researcher and now director of the Centre for Reading and Language Development at Tufts University in Boston, Professor Wolf has come across all too many cases of children who were not nurtured in this way.
Through the example of her dyslexic son Oliver, she has become interested in another group: those who struggle to read, despite involved parents and average or high intelligence.
Proust and the Squid is, in her words, "two parts science; one part personal observation". It covers all aspects of reading, from the history of how the Sumerians learnt to read to what happens in the brain when each individual child learns to read - or fails to. The reference to Proust, a French author, links to the evocative power of reading, while the squid stands for the neuroscience.
Early studies of the squid showed how brain cells (neurons) fire and transmit to each other, adapting, repairing and compensating when things go wrong.
In the same way, the invention of reading has required our brains to connect up older structures for vision and language to learn this new skill. As Stephen Pinker, the cognitive neuroscientist, put it: "Children are wired for sound but print is an optional accessory that must be painstakingly bolted on."
It's perhaps Professor Wolf's personal interest in dyslexia that makes the final section of her book the most interesting of all. She has a missionary zeal to avoid the human tragedy of intelligent people made to feel stupid because they find reading difficult. Even though they find it difficult, dyslexic children will learn to read, she says, "more laboriously and slowly, but they will learn", as the right hemisphere of their brain learns to take on the role played by the left in normal readers.
She calls for early screening and diagnosis and intensive intervention, using as many different approaches as possible. And she wants to see far more recognition of the compensatory creative skills that many dyslexics have - the list includes Leonardo da Vinci, Picasso, Albert Einstein and one in three entrepreneurs - probably because they are making greater use of their right hemisphere, which specialises in spatial skills.
At what age should children be explicitly taught to read? Professor Wolf is firmly against those who brandish flash cards in front of their babies in an attempt to give them a step up to early reading. "Five to seven is the right age," she says.
"People trying to teach true reading before this are in danger of doing a disservice to the biological schedule." Only about 5 per cent of children learn to read before the age of five without pressure to do so.
When teachers start to teach reading, Professor Wolf supports as active and rich an approach as possible, to match the richness and complexity of the reading circuits in the brain. Simply using phonics is not for her.
"Why is there this unnecessary debate between phonics and the whole language?" she asks. "The more children learn about a word, the better: the sounds, yes, but also visual recognition, meaning (and multiple meanings), connections, different grammatical uses . The more children learn about a word, the better and faster they will read it. Let the very young play with language, through jokes and rhymes.
"Reading never just happens. The wise teacher leaves nothing to chance and doesn't leave children to infer. Forty per cent of our children don't infer."
Professor Wolf ends our conversation, as she ends the book, voicing concern about the state of reading at a time when books are at increasing threat from the internet. As she says, during the seven years she took to write the book: "Reading changed beneath my sons' fingers."
What are the implications of digital culture for our capacity to read inferentially and analytically, she wonders? Will the immediacy and volume of information make us more intelligent as a species or short-circuit our deeper understanding of knowledge and our creativity, or both?
Her tentative answer is that children and teachers of the future should not have to choose. Just as children learn to switch between two or more oral languages, we can also teach them to switch between different presentations of written language and different modes of analysis. But as a lover of the slow pleasures of literature, she is doubtful whether this will happen.
Proust and the Squid: The Story and Science of the Reading Brain by Maryanne Wolf is published by Icon Books UK and is on sale now.