What's currently known about what causes autism resembles a pile of broken jigsaw pieces and no lid. While there is strong evidence that the root cause is genetic - studies of identical twins (who share exactly the same genetic material) show that if one of a pair has autism, there is a 60 per cent chance the other has too - there are precious few pieces on the board to identify which genes are relevant, or to link them to autistic behaviour.
The first definitive genetic link was reported in May this year by scientists from the University of Chicago who found autistic children were far more likely to inherit a shortened form of a DNA sequence that sparks off the production of the powerful brain messenger, serotonin.
A variety of other possible causal agents such as vaccines, viruses, food allergies and hostile immune reactions have been suggested, but geneticists reckon at least 90 per cent of autism cases result from genetic abnormalities which somehow cause fundamental changes in brain development.
A key problem for all investigators, says Dr Simon Baron-Cohen, lecturer in psychopathology in the Department of Experimental Psychology and Psychiatry at the University of Cambridge, and an expert in autism, is that despite common behaviours, people with autism do not appear to share brain abnormalities. "When you look at the brains of people with autism, you find evidence of all sorts of different abnormalities in different parts of the brain. There's not a single pattern."
One recent finding is that autistic brains are larger and heavier than normal brains and have far more neural connections. These connections are the brain's wiring and babies are born with an abundance. While in normal children, connections are gradually pruned down as functions such as language, numeric ability, memory and sensory input become specialised in particular parts of the brain, post-mortem studies of the brains of autistic people show that these areas of dense connections remain through life.
These extra connections may be a key factor in the way autistic people perceive the world, says Baron-Cohen, explaining, for example, the capacity they have for fine detail to the detriment of broader understanding. Ask an autistic child to draw a building and you might get all the bricks but no real building. Ask for a person and you may get an intensely detailed piece of jewellery but a facial void. The detail, says Baron-Cohen, seems to get in the way of seeing the whole. "If the way autistic people draw is how they actually perceive things, it may be that their attention to detail is at too great a level. What is useful in life is not attending too much to the detail in all situations but gathering as much as you need and then stepping back to look at the larger picture - autistic children seem not to be able to do this."
Dr Baron-Cohen says that this abnormal attention to detail could explain other autistic features such as the savant abilities of about 5 per cent of autistic people, and the anxiety expressed when things are changed.
"If the brain in an autistic individual is gathering very intensive detail, then they may want to repeat events or situations to check whether that detail is changing, whereas the rest of us can tolerate a lot of variation. It doesn't matter whether a person is wearing different clothing when we see them, they are still the same person. For a person with autism, if the details are changed it may be more difficult to identify this as a predictable event.
"If you came home and found your house had moved to somewhere else down the road - it would be very unsettling. For an autistic person even a small change to the environment might be just as unsettling."
Another loose piece in the causative jigsaw pile is that from infancy, autistic children attend differently. Normally, developing babies, says Baron-Cohen, focus on faces, particularly eyes, and will follow the direction of another person's gaze. Known as joint attention, this appears to be an in-built mechanism for getting out of our own perspective and tuning in to what people think or know, he says. But autistic children don't do this.
"Joint attention appears to be very important for developing communication, and for monitoring what other people are feeling or thinking. It may be that if you leave a child without joint attention, all sorts of problems arise from the fact that they basically miss out on learning from other people."
Barring gene technology, says Baron-Cohen, developing ways of stimulating joint attention early in life could be the best hope for helping autistic people learn to live more easily in world that's literally on a different wave length. HILARY BOWER