Autism is more often diagnosed in boys
Scientists have produced the most compelling evidence to date that genetics play a key role in autism.
They highlighted tiny genetic changes that appear to have a strong impact on the likelihood of developing autism and related conditions.
The changes influence genes which help form and maintain connections between brain cells.
The Nature study highlighted one common genetic variant which, if corrected would cut cases of autism by 15%.
Previously, other genetic variants have been linked to autism, but they are all relatively rare.
Dr Raynard Kington, of the US National Institutes of Health, which funded the research, said: "These findings establish that genetic factors play a strong role in autism spectrum disorder (ASD).
"Detailed analysis of the genes and how they affect brain development is likely to yield better strategies for diagnosing and treating children with autism."
People with ASD, which include autism and Asperger's syndrome, have problems with social interaction, poor communication skills and tend to engage in repetitive behaviours.
In the latest studies researchers scanned the human genome for small differences between people who have an ASD, and those who do not.
The largest study, led by the Children's Hospital of Philadelphia, focused on more than 10,000 people.
It found several genetic variants commonly associated with ASD, all of them pointing two specific genes found on chromosome 5 which control production of proteins which help cells stick to each other, and make nervous connections.
A bead chip used to analyse human DNA en masse
One variant, linked to a gene called CDH10, was so common - present in over 65% of cases of autism - that the researchers calculated that fixing it would cut the number of autism cases by 15%.
They also linked ASD rather less strongly to a group of about 30 genes which produce proteins that play a key role in enabling brain cells to migrate to correct places, and to connect to neighbouring cells.
Other genetic changes pinpointed by the Pennsylvania team occurred in genes involved in a cellular waste system which probably ensures these "adhesion" proteins are kept in working order.
Lead researcher Dr Hakon Hakonarson said the genetics of ASD was likely to be complex.
He said: "Because other autism researchers have made intriguing suggestions that autism arises from abnormal connections among brain cells during early development, it is very compelling to find evidence that mutations in genes involved in brain interconnections increase a child's risk of autism."
But he added: "There are going to be many genes involved in causing autism.
"In most cases, it's likely that each gene contributes a small amount of risk, and interacts with other genes and environmental factors to trigger the onset."
Similar findings were reported in separate studies published in Annals of Human Genetics and Molecular Psychiatry.
Professor Simon Baron-Cohen, an autism expert at the University of Cambridge, said 133 genes had now been linked to the condition, and much work was needed to piece together how they interacted with each other and the environment.
He said: "The puzzle is slowly being pieced together, and the science of autism is accelerating in promising ways."
The National Autistic Society said the exact causes of autism were unknown.
In a statement, the society said: "There is evidence to suggest that genetic factors are responsible for some forms of autism.
"However, the difficulty of establishing gene involvement is compounded by the interaction of genes and by their interaction with environmental factors.
"Various studies over many years have sought to identify candidate genes but so far inconclusively."