A baby's smile gives a new mother a high like a drug hit and is equally addictive, a study of the female brain shows.
Neural scans on women help explain the extremely strong bond between mother and baby by showing how images of the infant affect the brain.
A study involving 28 first-time mothers shows that when a woman looks at a photo of her baby smiling the reward centres of her brain light up.
These regions, called the substantia nigra, the striatum and the frontal lobe, are involved in emotion processing, cognition and behavioural outputs.
Lead researcher Dr Lane Strathearn, a Queenslander now based in the US, said these areas had also been activated in experiments associated with drug addiction.
"It may be that seeing your own baby's face is like a natural high," said Dr Strathearn, who is now based at the Baylor College of Medicine in Texas.
The study, published in the journal Paediatrics, showed this maternal activation was strongest with smiling faces.
Photos of babies crying did not evoke the same brain response.
In fact, there was little difference in mothers' brains when they saw their own babies' crying face compared to that of an unknown child, Dr Strathearn said.
The findings could help scientists explain the inner workings of that critical mother-infant bond.
"Understanding how a mother responds uniquely to her own infant, when smiling or crying, may be the first step in understanding the neural basis of mother-infant attachment," he said.
It could also help shed light on how the bond fails to form in some cases.
"The relationship between mothers and infants is critical for child development," Dr Strathearn said.
"For whatever reason, in some cases, that relationship doesn't develop normally and neglect and abuse can result, with devastating effects on a child's development."
