Scientists believe they have located the part of the brain key to forging the parental instinct to care for and nurture an infant, according to a study published on Wednesday.

This discovery helps answer the evolutionary question of why people view babies as special and could help doctors better identify people suffering from postnatal depression, researchers at the University of Oxford said in the journal PLoS One.

"It is important because there has to be a reason why we look after our kids in general to make sure our species survives," said Morten Kringelbach, a neuroscientist, who co-led the study. "This is an idea that goes back to Darwin."

Kringelbach and his colleague Alan Stein showed how a region of the brain called the medial orbitofrontal cortex lights up to faces of infants but not to adults.

Scientists believe this area - located just above the eyeballs and connected to the area important for recognizing faces - is key to controlling emotions, Kringelbach said.

To do this, they used imaging scans to measure brain activity in volunteers asked to hit a button when a cross on a screen in front of them changed colour. In between, images of unfamiliar infant and adult faces flashed on the screen.

INSTINCTIVE SIGNAL

There was no reaction to the adult faces but the infant images spurred a high level of activity in the brain within a second, an instinctive signal that may tell us babies are special, the researchers said.

"We think that the early response is guiding us to treat infants as special," Kringelbach said in a telephone interview. "It happens so fast that almost without a doubt there is no conscious control over that."

The parental response to infants was similar among men, women and volunteers in the study who didn't have children, providing evidence that this reaction is innate, he added.

The findings could help doctors identify the 15 percent of women and three percent of men in the developed world who suffer from postnatal depression, the researchers said.

Measuring this response in men and women could help predict people at highest risk of developing the condition that can make new parents ignore their babies, Kringelbach said.

"This marker may not be as strong in people with post-natal depression," he said. "If that is true we could use that to look at people to see if that response is dimmed and if they are more likely to get postnatally depressed."