In a Novel Theory of Mental Disorders, Parents’ Genes Are in Competition
Their idea is, in broad outline, straightforward. Dr. Crespi and Dr. Badcock propose that an evolutionary tug of war between genes from the father’s sperm and the mother’s egg can, in effect, tip brain development in one of two ways. A strong bias toward the father pushes a developing brain along the autistic spectrum, toward a fascination with objects, patterns, mechanical systems, at the expense of social development. A bias toward the mother moves the growing brain along what the researchers call the psychotic spectrum, toward hypersensitivity to mood, their own and others’. This, according to the theory, increases a child’s risk of developing schizophrenia later on, as well as mood problems like bipolar disorder and depression.
In short: autism and schizophrenia represent opposite ends of a spectrum that includes most, if not all, psychiatric and developmental brain disorders. The theory has no use for psychiatry’s many separate categories for disorders, and it would give genetic findings an entirely new dimension.
“The empirical implications are absolutely huge,” Dr. Crespi said in a phone interview. “If you get a gene linked to autism, for instance, you’d want to look at that same gene for schizophrenia; if it’s a social brain gene, then it would be expected to have opposite effects on these disorders, whether gene expression was turned up or turned down.”
The theory leans heavily on the work of David Haig of Harvard. It was Dr. Haig who argued in the 1990s that pregnancy was in part a biological struggle for resources between the mother and unborn child. On one side, natural selection should favor mothers who limit the nutritional costs of pregnancy and have more offspring; on the other, it should also favor fathers whose offspring maximize the nutrients they receive during gestation, setting up a direct conflict.
The evidence that this struggle is being waged at the level of individual genes is accumulating, if mostly circumstantial. For example, the fetus inherits from both parents a gene called IGF2, which promotes growth. But too much growth taxes the mother, and in normal development her IGF2 gene is chemically marked, or “imprinted,” and biologically silenced. If her gene is active, it causes a disorder of overgrowth, in which the fetus’s birth weight swells, on average, to 50 percent above normal.
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.