Biologists call this gene imprinting an epigenetic, or “on-genetic,” effect, meaning that it changes the behavior of the gene without altering its chemical composition. It is not a matter of turning a gene on or off, which cells do in the course of normal development. Instead it is a matter of muffling a gene, for instance, with a chemical marker that makes it hard for the cell to read the genetic code; or altering the shape of the DNA molecule, or what happens to the proteins it produces. To illustrate how such genetic reshaping can give rise to behavioral opposites  the yin and yang that their theory proposes  Dr. Crespi and Dr. Badcock point to a remarkable group of children who are just that: opposites, as different temperamentally as Snoopy and Charlie Brown, as a lively Gaugin and a brooding Goya.

Those with the genetic disorder called Angelman syndrome typically have a jerky gait, appear unusually happy and have difficulty communicating. Those born with a genetic problem known as Prader-Willi syndrome often are placid, compliant and as youngsters low maintenance.

Yet these two disorders, which turn up in about one of 10,000 newborns, stem from disruptions of the same genetic region on chromosome 15. If the father’s genes dominate in this location, the child develops Angelman syndrome; if the mother’s do, the result is Prader-Willi syndrome, as Dr. Haig and others have noted. The former is associated with autism, and the latter with mood problems and psychosis later on  just as the new theory predicts.

Emotional problems like depression, anxiety and bipolar disorder, seen through this lens, appear on Mom’s side of the teeter-totter, with schizophrenia, while Asperger’s syndrome and other social deficits are on Dad’s.

It was Dr. Badcock who noticed that some problems associated with autism, like a failure to meet another’s gaze, are direct contrasts to those found in people with schizophrenia, who often believe they are being watched. Where children with autism appear blind to others’ thinking and intentions, people with schizophrenia see intention and meaning everywhere, in their delusions. The idea expands on the “extreme male brain” theory of autism proposed by Dr. Simon Baron-Cohen of Cambridge.

“Think of the grandiosity in schizophrenia, how some people think that they are Jesus, or Napoleon, or omnipotent,” Dr. Crespi said, “and then contrast this with the underdeveloped sense of self in autism. Autistic kids often talk about themselves in the third person.”

Such observations and biological evidence are hardly enough to overturn current thinking about disorders as distinct as autism and schizophrenia, experts agree. “I think his work is often brilliant,” Dr. Stephen Scherer, of the University of Toronto and the Hospital for Sick Children, said by e-mail message of Dr. Crespi. At the same time, Dr. Scherer added, “For autism there will not be one unifying theory but perhaps for a proportion of families there are underlying common variants” of genes that together cause the disorder.