Other researchers were more cautious, saying that the genetics of rare mutations was not yet well enough understood to make conclusive statements about their effect on the behavior of specific genes.

“This is a great beginning, and I’m impressed with the work, but we don’t know the cause of these rare mutations, or even their levels in the general population,” said Aravinda Chakravarti of the Institute of Genetic Medicine at the Johns Hopkins University Medical School, who was not involved in the studies. “I’m not saying it’s not worth it to follow up these findings, but I am saying it’s going to be a hard slog.”

The three research teams took a similar approach, analyzing genetic material taken from blood samples of families in which parents who have no signs of autism give birth to a child who develops the disorder. This approach gives scientists the opportunity to spot the initial mutations that accompany the condition, rather than trying to work though possible genetic contributions from maternal and paternal lines. In all three studies, the researchers focused on rare genetic glitches called de novo mutations.

De novo mutations are not inherited but occur spontaneously near or during conception. Most people have at least one, and the majority of them are harmless.

In one of the new studies, Dr. Matthew W. State, a professor of genetics and child psychiatry at Yale, led a team that looked for de novo mutations in 200 people who had been given an autism diagnosis, as well as in parents and siblings who showed no signs of the disorder. The team found that two unrelated children with autism in the study had de novo mutations in the same gene — and nothing similar in those without a diagnosis.

“That is like throwing a dart at a dart board with 21,000 spots and hitting the same one twice,” Dr. State said. “The chances that this gene is related to autism risk is something like 99.9999 percent.”