A single gene may play a key role in determining whether infants bond with their mothers, say scientists.
Is bonding in our genes?
Tests on mice have found that knocking out the gene can transform the way newborn mice relate to their mothers.
The gene regulates a receptor in the brain that responds to opioids, ranging from painkillers to heroin.
Writing in the journal Science, researchers said it also enables infants to find mums "rewarding" and bond with them.
Scientists from the CNR Institute of Neuroscience in Rome bred two groups of mice.
The first had both copies of their µ-opioid receptor gene knocked out. The second did not.
Mice from both groups were taken away from their mothers after just a few days and placed in a new environment.
The normal mice screamed incessantly. They screamed about half as much when they were given something that smelled of mum.
However, the mutant mice showed few signs of missing mum, hardy screaming at all.
Both sets of pups were then given the option of returning to their mum or going to another mum's nest.
All of the normal mice returned home. However, just a third of the mutant pups did likewise.
"The pups weren't able to discriminate between moms," said Francesca D'Amato, who led the research.
Jaak Panksepp, a neuroscientist at Bowling Green State University in Ohio, welcomed the study.
He said it provided "robust" evidence that "a pup needs opiate activation in order to find its mother rewarding."
The scientists said the findings may have implications for children with autism.
These children do not form normal social bonds with their parents and these findings suggest that a problem in their µ-opioid receptor gene may be partly to blame.
However, Thomas Insel, who works at the US National Institute of Mental Health, said the findings don't necessarily apply to humans.
"You can think of [human] attachment as love," he told Science.
"It's selective and enduring. Mice don't have selective attachments."
The UK's National Autistic Society suggested autism was a complex condition.
"There are probably underlying genetic bases to autism but recognises that many other factors could be linked," said a spokeswoman.
"Autistic spectrum disorder is a lifelong developmental disability characterised by difficulties with social interaction, communication and imagination."