Love is in some measure the influence of a brain chemical called oxytocin and fluctuating levels of vasopressin. Aggression and depression are a measure of serotonin levels.
By altering a single gene that affects vasopressin, scientists can turn philandering mice into faithful, devoted mates.
Eliminate four genes that affect levels of oxytocin and estrogen, and mice cannot recognize their friends or their enemies.
Make just one genetic change in how a brain hormone such as serotonin is controlled and researchers also can eliminate or elevate some kinds of aggression -- at least in mice.
By examining images of the brains of identical twins, UCLA neurologist Paul Thompson and his colleagues have shown how such genes shape the anatomy of the human brain. The more genes people have in common, the more their brains are alike, the researchers discovered.
This strong family resemblance may explain why neural diseases and mental disorders, including schizophrenia and some types of dementia, run in families. It suggests one way parents may pass on personality traits to their children.
If genes bind family members to one another, the nuances of experience are what set families' members apart, according to the American Society of Human Genetics.
The way a mother holds and strokes her child can chemically alter the expression of some of the child's genes and temper their effects, said psychiatrist Glen Gabbard at Baylor College of Medicine, who studies how biology and the environment affect personality disorders.
"The quality of maternal interaction may override the genetic predisposition to criminal behavior," Gabbard said.
Such biochemical changes in how a gene behaves can, in turn, be inherited, even though they do not actually change the structure of the gene itself.
Molecular biologists at the University of Wisconsin and King's College London last year discovered a gene linked to antisocial behavior whose activity seems to be controlled by the intersection of heredity and home life.
One variant of this gene, which can affect levels of serotonin, appears to be triggered by the experience of child abuse.
"We have come to the realization that there is no dichotomy between genes and environment," said UCLA clinical psychiatrist Regina Palley. "They both interact at a biological level."
Brain scans and genetic pedigrees have become a staple of death penalty proceedings, said Alison McInnes, director of the laboratory of neurobehavioral genetics at Mount Sinai School of Medicine in New York.
As an expert defense witness in the trial of Yosemite Park killer Cary Stayner, she testified during the sanity phase of his death penalty proceeding. She tried to show how genes influenced the state of mind that led him to murder three tourists and a nature guide in 1999. A second scientist testified that a brain-imaging scan showed abnormalities that might have made Stayner more prone to violent impulses.
Even so, the jurors decided that he was legally sane and sentenced him to die.
On death row in Georgia, convicted murderer Stephen Mobley appealed his death sentence by arguing that he had a right to be tested for genes linked to impulsive and uncontrollable violence. The U.S. 11th Circuit Court of Appeals disagreed. He is awaiting execution.
Forensic neurobiologist Paul Rossby in El Paso has testified as an expert witness in seven death penalty cases, in which he measured serotonin and dopamine activity in the brains of the accused.
"We are able to provide a jury with plausible explanations for the most inexplicable acts of horror," Rossby said. "This is the wave of the future."
If researchers can better understand how biology makes the brain go awry, they may one day predict who is predisposed to aggressive, violent behavior and, perhaps, find ways to intervene long before any harm can occur.
Today, all 50 states are building computer data banks of DNA sequences collected from convicted felons. At least 20 states allow scientists to use those data banks for genetic research.
"The genetic markers [they find] are going to be far more useful in criminal justice and forensics than they will be useful for medicine," said sociologist Troy Duster of New York University and UC Berkeley.
Some legal experts, sociologists and ethicists worry that medical techniques developed to treat behavior will instead become tools of control, transforming crime and punishment.
In the past, scientific theories of deviant behavior were used to justify abuses, from U.S. eugenics programs to sterilize the "feeble-minded" to the Nazi genocide of the genetically "unfit."
Biological research on human aggression and violence remains controversial today, in part because genetic discoveries about human behavior so often have been wrong or oversimplified.
In recent years, scientists have offered genetic explanations for homosexuality, smoking, divorce, suicide, schizophrenia, alcoholism, shyness, political liberalism, intelligence and criminality.