Amid all the controversy over the 12A classification awarded to the latest Batman blockbuster, The Dark Knight, one vital piece of evidence has yet to be addressed. And it could be the most crucial of all.

While there is understandable concern about children being upset by  -  or acting out  -  screen violence, recent findings by scientists about the long-term effect of such material on the brain is, for me, by far the most shocking aspect.

In one study, experts found that changes occur in the brain functions that control impulse and aggression when children see scenes of violence on screen. And there is growing evidence that this change can lead to violent behaviour in the future.

Disturbing: Batman and The Joker in a scene from The Dark Knight

Researchers monitored brain activity in a group of children while they watched a variety of film scenes  -  ranging from a bloody boxing match in the Sylvester Stallone film, Rocky IV, to images of baby animals.

When watching Rocky IV, a section of the children's brains was activated that is also believed to hold long-term memories of traumatic events. It is where rape victims store memories of being attacked, for example, and where traumatised war veterans store their experiences in battle.

The study's lead author, Professor John Murray, testified before the U.S. Senate that the brain 'treats entertainment violence as something real and stores this violence as long-term memory'. He added: 'The brain is treating it as serious stuff.'

Many parents labour under the misapprehension that by accompanying their children to the cinema or by sitting with them in front of the television they can counter the effects of violence on them. Others, unrestrained by common sense, argue that by teaching children 'media literacy' - in other words explaining that screen violence is not real  -  will immunise them from any effects.

But new neurological evidence shows that children are unable to distinguish between fictional and actual violence and indicates that parents can do nothing to minimise the long-term effects of what their child is seeing just by being present or discussing it with them.

Comparing the brain activity of non-aggressive adolescents with those diagnosed with disruptive behavioural disorders while they watched moderately violent scenes provided further insights.

In another study, adolescents with DBD showed less activity in their frontal lobe  -  the area responsible for decision-making, behaviour, impulse control and attention  -  than those who had had little or no exposure to such material before.

Even more interesting was follow up research which found that watching screen violence had changed the frontal lobe brain function of the 'normal adolescents' to be more like that of the children with DBD.

The entertainment industry claims no two films  -  or indeed video games  -  will affect the same people in the same way and argue that you can't generalise.

Video games may exert an even more powerful effect on a child and now is there further evidence to show precisely what happens to a child's brain while he or she is shooting the 'bad guys'.

This study, published in the journal Human Brain Mapping, described how brain areas associated with emotional and appropriate social responses are suppressed. With every new round on the video game, the players reacted more violently but with less emotion to the threat of violence conditioning their brains to respond this way in real life.

The younger the child, the more easily the size, structure and function of their brain can be altered permanently as a direct result of what they are exposed to.

A good example of this is the finding that learning a musical instrument by the age of 12 will actually make the left temporal region of a child's brain larger. This gives them a better verbal memory and vocabulary in later life.

Equally, the brain size and function of children raised without nurturing or love is also different to those raised in a warm, loving environment. What we must take from these and many other examples is that children's brains are in transition and it is important that we control  -  censor  -  what they experience in order to provide the best conditions for their development.

However, the evidence of the potential danger posed to a child's brain development by these violent films does not seem to feature in the decisions made by those entrusted to look after their interests.

Earlier this year in removing bans on certain 'video nasties', the British Board of Film Classification reportedly stated that while they were tasteless, there was no evidence the films caused harm to viewers. 'Times shift, attitudes change,' they said.

Social mores may change  -  the brain function of children and young people does not.

In the U.S., six of the country's most prominent medical groups, including the American Medical Association, are so concerned about the lack of appropriate screen censorship that they appeared before a U.S. Congressional Public Health summit which said: 'The conclusion of the public health community, based on over 30 years of research, is that viewing entertainment violence can lead to increases in aggressive attitudes, values and behaviour.'

By giving this violent Batman a 12A classification, the BBFC exemplifies the preoccupation that those in authority have for not offending or disappointing when that is part of their role, for not appearing authoritarian when we need them to be, and for always wanting to be 'in touch' with youth culture when their job is simply to protect the young.

• Dr Aric Sigman is an Associate Fellow of the British Psychological Society, Fellow of the Royal Society of Medicine and author of Remotely Controlled (Vermilion, 2007).