* Early last century most behaviourists believed children were creatures to be shaped and moulded by adults
* The Swiss psychologist Jean Piaget overturned this view. His work on how children think continues to influence research and to shape what goes on in primary schools
* By age two a child has made one hundred trillion synapses - the electrical connections that transmit messages between mind and body and within the brain itself
* By three, a child has a spoken vocabulary of about 1,000 words, rising to between 8,000 and 14,000 for the average six-year-old
In E Nesbit's classic story, Five Children and It, a dreadful thing happens to the baby known as the Lamb. He turns into an adult, not gradually, but instantly. "Before the horrified eyes of its brothers and sisters, the Lamb suddenly and violently grew up. The Baby's face changed first... Most terrible of all, a little dark moustache appeared on the lip of the one who was still a two-year-old baby in a linen smock and white open-work socks."
The transformation continues until the Lamb is a "very proper-looking young man in flannels and a straw hat". But has his mind grown up with his body? "If his inside senses are grown up too, he won't stand us looking after him; and if he is still a baby inside of him how on earth are we to get him to do anything? And it'll be getting on for dinner-time in a minute," says his sister Anthea.
The children don't like the adult Lamb, and resolve, once he is restored to toddlerhood, to sort out his upbringing. "If he grows up in the usual way there'll be plenty of time to correct him as he goes along," says his brother Robert. "The awful thing today was his growing up so suddenly.
There was no time to improve him at all."
When E Nesbit was writing in 1902, ideas about child development were dominated by the behaviourists and the Freudians. (You could be a victim of your conditioning or your subconscious.) Mostly they disagreed, but they did share one attitude encapsulated by Robert's "There was no time to improve him": children were creatures to be shaped and moulded by adults.
As the leading behaviourist John B Watson put it in 1930: "Give me a dozen healthy infants, well formed, and my own specified world to bring them up in, and I'll guarantee to take any one at random and train him to become any type of specialist I might select - doctor, lawyer, merchant-chief and, yes, even beggar-man and thief."
The Swiss psychologist Jean Piaget (1896-1980) overturned this view of children as passive playthings of adults, arguing that they were active agents in their own development who learned through their attempts to make sense of the world. Piaget's work continues to influence research and shape what goes on in primary schools. But, recently, psychologists have claimed he underestimated children's abilities, probably because he overestimated their language skills. When his ingenious cognitive tests are revised to key into infants' experience (for example, asking them to sort toy animals rather than pictures of flowers), children are found to be able to reason and empathise earlier than Piaget realised.
Next on the child development list of stars comes Lev Vygotsky, born, like Piaget, in 1896. He recognised the importance of social interaction for children, specifically the role of adults as teachers. An influential report (by the Institute of Education, see resources) on effective pedagogy in the early years released last year backed Vygotsky's ideas, urging nursery staff to teach children by modelling appropriate language and behaviour, sharing intelligent conversations, asking questions and using play to motivate and encourage.
Many influential scientists have followed in the footsteps of Piaget and Vygotsky. Child development is a popular area for research, but it can be frustrating, says Dr Jeni Riley, head of early childhood and primary education at the Institute of Education in London. Too many researchers still study children from just one angle - be it psychological, social, behavioural or genetic. "Sometimes you wish they would just talk to one another," she says. Only a multi-perspective approach has any chance of understanding the complex interactions that will determine whether the Lamb grows up a spoiled brat or a decent chap.
The end of nature versus nurture
Nature versus nurture is dead. Long live G+E, otherwise known as genetics plus environment. Neurobiological, behavioural and social scientists are striving to understand what makes for a good start in life. Their work has deepened our understanding of how a child's genes affect its responses to its world, and how its environment affects the expression of those genes.
The question today is not whether early experiences matter, but how they shape individual development, and help - or hinder - healthy growth.
Recognition of the drastic effect poverty, for example, can have on the developing child lies behind social programmes such as Sure Start.
Love is all
In the 1920s and 1930s, children growing up in orphanages were "failing to thrive". Many died and others grew up stunted. Observers were puzzled because the youngsters were getting an adequate diet. The reason - that young humans need to love and be loved as well as eat - now seems obvious.
Clearly it was not so in 1952, when psychiatrist John Bowlby told the World Health Organisation that a child had an innate and primary need to become "attached" to a carer. Previously, such love had been rated less important than food, water and warmth.
Much work since has investigated the quality and nature of these attachments. Bowlby emphasised the maternal bond, but later research eased the guilt of working parents by showing that children can form secure attachments to several carers. Cultural influences also play a part. In 1985, for example, researchers found that two-thirds of infants in the US had formed a secure attachment to a parent, compared to just one in three in Germany. The German tradition of pushing children towards independence shortly after they had sampled their first birthday cake was considered the likely reason.
Post-Bowlby researchers have classified children as being "securely attached", "insecure-avoidant" or "insecure-resistant". A new category - the "insecure-disorganised" child - has recently been added to the list.
These few young people have usually endured maternal depression, loss, divorce or similar trauma. A long-term follow-up of disorganised infants in 1998 found, not surprisingly, that they suffered serious behaviour problems at school. Neuroscience has backed up the psychologists. Work with monkeys reared in isolation is helping scientists understand that lack of attachment in infancy affects the development of emotional areas of the primate brain.
Young humans seem equally vulnerable. Their brains are far more impressionable ("plastic") than adults'. This makes them more open to learning and enriching influences, but also more susceptible to long-term developmental problems if the world turns nasty. For example, children adopted from Romanian orphanages of the Ceausescu era were found to have high levels of stress hormones more than six years later.
According to Dr Riley, the extraordinary power of the infant brain is only beginning to be explored. We are at the start of learning how this "astonishing potential" can be used to stimulate and develop children.
A baby's brain is about one quarter the weight of an adult's and works at only one sixteenth the speed. But it is buzzing. The electrical connections that are to transmit messages between mind and body and within the mind itself have to be made. These are known as synapses and we need a lot of them. During what neuroscientists call "the exuberant period" of early childhood, the cerebral cortex, the outer surface of the brain that regulates complex thinking, can produce an amazing two million synapses a second.
Everything an infant does, from shaking a rattle to opening its first book, excites certain neural circuits - sensory, motor, emotional, cognitive - and leaves others unused. Those that see a lot of action grow stronger.
By age two a child has made 100 trillion synapses. By age five its brain has reached 90 per cent of adult weight and will already be shedding some of these connections as it adjusts itself to its environment. Only about half survive into adulthood. This loss, known as "pruning", is crucial to a child's development, say neuroscientists, because it gets rid of superfluous mental wiring. Language provides a good example of pruning.
Babies have an acute ear for differences in sounds. Experiments with English, Japanese and Chinese infants show that until about the age of one they can all distinguish between a "k" and a "l" sound. After that, just when they are starting to speak themselves, the Chinese and Japanese children lose the ability. Their languages do not distinguish between the sounds, so they don't need to.
Talk, talk, talk
Experiments with rats have shown that stimulation encourages the creation of synapses. A neglected rodent in a dull cage will have a far simpler neural network than a rat well supplied with toys and attention.
A baby's brain thrives on sensitive, responsive caring. Scientists have not come up with any way of enhancing this natural process, with one exception - language. Infants who are talked to, read to, and otherwise engaged in lots of verbal interaction develop slightly more advanced linguistic skills than those who are not.
Language development and cognitive development are undoubtedly linked - though scientists disagree on how - so talking and listening to children is likely to help them greatly in their critical brain-building years. (That said, research done with the Kaluli tribe in a remote area of Papua New Guinea found that children learned to speak perfectly normally, even though adults did not talk to babies, considering them as "having no understanding".) Children have many strategies to help them in the monumental task of learning language. "Fast mapping" is one. In 1994 scientists showed five objects to babies aged 16 to 20 months. Four of the objects were familiar while the fifth was new - a garlic crusher, for example. The children were each asked for one of the familiar items, such as a ball, and once they had pointed to it, they were asked for "the lep". Fast-mapping children knew that new object equalled new word and pointed to the garlic crusher.
Such strategies mean that, by three, a child has a spoken vocabulary of about 1,000 words, rising to between 8,000 and 14,000 for the average six-year-old, and is making its first stabs at grammar and syntax. This grammatical precociousness, which may have deceived Piaget, led the linguistic theorist Noam Chomsky to suggest, controversially, that some language skills are passed on genetically.
But another theory holds that children have a strong predisposition to learn a language rather than an innate knowledge of one. For example, babies are born able to discriminate between phonemes, but have to learn, like the Chinese and Japanese infants, which ones are important in their language.
Specialists have believed that children go through "critical periods" - times in their lives when they can acquire skills particularly easily. For example, in music, perfect pitch seems to develop only in musicians who start training before age seven. And in languages the learning window appears to start closing around age five, finally shutting completely at puberty. Any French, German or foreign languages learned after that will almost always will be spoken with an English accent.
But the theory is under fire. Biologists prefer to see children's development as a resilient and flexible process, not fixed and unalterable.
Similarly, some early educators would rather talk of the "critical experiences" that are vital to school success, mental health and social development, and the "teachable moments" when a child is motivated to learn.
Jean Piaget's work on children's thinking has left deep roots in the classroom. Only last year, researchers from King's College London used his tests to help teachers accelerate children's cognitive development.
Piaget said young people moved through three stages of development, arriving at the fourth - the "formal operations" stage - in adolescence.
This stage involves the logical use of symbols related to abstract thought.
Latterly it has been shown that most adults never reach it. According to Piaget, each stage involves a qualitative change in the way a child reasons. For example, most under-sevens at the "pre-operational stage" will respond incorrectly when asked what is the real colour of a piece of white paper placed behind a blue filter.
This concept of "stages" has been challenged by an approach to child development that draws parallels between the human brain and a computer.
The maturing nervous system is the hardware and a child's learning strategies are the software. This "information processing" approach sees a gradual increase in children's powers of thinking and has encouraged teachers to focus on what youngsters can do, whatever their age, rather than searching for signs that they are ready to move on to the next stage.
Nevertheless, children do generally progress as Piaget observed, and recent studies of what is happening inside the brain are beginning to identify specific structures that may need to be up and running before youngsters can "pass" his tests. Children at the pre-operational stage find it difficult to make logical deductions (if the blue rod is shorter than the green rod and the green rod is shorter than the yellow rod, which is the shortest rod?) and it seems that the development of the pre-frontal cortex is crucial to these tasks. Fortunately for children presented with such questions, researchers have noted that a sense of humour usually starts to develop in the first year of life.
Back on the agenda
Teacher training went through a "rocky stage" when little attention was paid to child development, says Dr Riley. But over the past few years training programmes have become much more balanced and the subject is taught within the classroom context.
"Everything I do, I do in relation to what happens in the classroom, because I believe passionately that if you don't understand how children learn you won't be able to support it effectively," she says.
Main text: Stephanie Northen. Photographs: Hulton. Additional research: Tracey Thomas
Next week: The unions and professional development