Good parenting could prevent antisocial behaviour in people genetically prone to aggression, findings in animals suggest.
Upbringing and genes may interact in aggression
Past studies have linked certain genes with aggressive behaviour in both humans and animals.
Dr Stephen Suomi from the US National Institutes of Health said studies in monkeys show a nurturing environment can buffer the effect of these genes.
His research will be published later this year in Biological Psychology.
Previous research has linked low levels of brain chemicals such as serotonin and monoamine oxidase A (MAOA) with aggression in humans and animals.
The levels of these chemicals are controlled by genes.
Scientists know some people have genes that make them susceptible to aggression and anti-social behaviour.
People hospitalised for depression and other psychiatric disorders have been found to have a genetic make-up that means they have lower levels of serotonin or MAOA, for example.
But environmental factors such as upbringing are also known to be important in behaviour.
Dr Suomi, from the National Institute of Child Health and Human Development, and colleagues studied a large population of wild rhesus monkeys, which are closely related to humans in terms of genes.
The monkeys were divided into groups based on their observed aggressive behaviours. About 5-10% of the monkeys showed excessive levels of violent, antisocial behaviour.
These aggressive monkeys had the least serotonin levels in the brain, while the least aggressive had the most.
As previously shown, the serotonin levels, which are controlled by genes, were linked to the monkey's genetic make-up.
In another study, Dr Suomi looked at the effect of rearing on young monkeys genetically prone to aggressive behaviour.
From birth to six months of age, the monkeys were reared either in a nurturing, supportive way by their mother or were left with their siblings to fend for themselves.
As expected, the monkeys left with their siblings had low levels of serotonin and displayed aggressive behaviour.
But the monkeys who had been reared by their mothers had normal levels of serotonin and displayed normal behaviour despite being genetically prone to aggression.
Dr Suomi said: "It's a gene-environment interaction. There's a buffering effect of good mothering.
"Good mothering is very important and may actually protect individuals who have genes that make them vulnerable," he said.
But he cautioned that the findings were in animals and could not be generalised to humans.
"We have to be careful about making comparisons across species. All we have demonstrated is an interaction between one particular gene and the environment.
"There could be many, many more," he said.
Dr Suomi is doing more research looking at how positive rearing might protect against aggression in monkeys genetically prone to this behaviour.
Nature and nurture
Professor Donald Pfaff, from Rockefeller University in the US, who has conducted similar research into aggression in animals, said more research was needed.
"It's a very big puzzle at the moment. There are over 50 genes that contribute to the arousability of the brain," he said.
Dr Clio van Velsen, consultant psychiatrist in forensic psychotherapy for East London and the City Mental Health Trust, said: "There is no longer this simplistic divide between nature and nurture.
"This need to attach is absolutely fundamental to human development. It sets the template to later developing.
"If you think of two babies born on the same day. One cries a lot and so people don't really want to pick him up a lot. Then you have the other baby who is one of those cutesy things.
"You can see that children create the environment that they are influenced by. That's where the genetics and the environment come together. Those two babies actually have different environments, but that's partly a function of who they are.
"You could argue that the genetic component that makes temperament difficult and hard to cuddle emphasises those elements of their personality that might later turn out to be difficult or aggressive," she said.