Children neglected in their early years are left with physical as well as psychological marks, research suggests.
The children played computer games with their mum or a stranger
Lack of a loving caregiver directly affects the body's production of hormones thought to be important for forming social bonds, a US team found.
Children raised in orphanages had lower levels of vasopressin and oxytocin than others, Proceedings of the National Academy of Sciences reports.
This was despite the children later being placed with stable families.
This suggests the effects may be lasting to some extent, the University of Wisconsin-Madison authors said.
They believe failure to receive typical care as a child can disrupt normal development of these hormonal systems which, in turn, can interfere with the calming and comforting effects that typically emerge between children and their caregivers.
Compared with the control group, the 18 four-year-old children raised in orphanages showed lower levels of vasopressin in their urine.
Researchers believe this hormone is essential for recognising individuals in a familiar social environment.
During an experiment, the children were asked to sit on the laps of either their mother (or adopted mother) or an unfamiliar woman and play an interactive computer game.
The game directed the children to engage in various types of physical contact with the adult they were sitting with, such as whispering or tickling each other and patting each other on the head.
This type of interaction between a child and his or her mother should normally cause a rise in oxytocin. This was seen in the family-raised children, but orphanage-raised children did not display the same response.
Lead researcher Dr Seth Pollak said: "It's extremely important that people don't think this work implies that these children are somehow permanently delayed.
"All we are saying is that, in the case of some social problems, here is a window into understanding the biological basis for why they happen and how we might design treatment."
The researchers added: "The present data provide a potential explanation for how the nature and quality of children's environments shape the brain-behavioural systems underlying complex human emotions."
Dr Julie Turner-Cobb, of Bath University, and Dr David Jessop, at Bristol University, recently carried out research showing children can be stressed out by their own mothers' emotional exhaustion.
Childcare helped to reduce stress (measured by a hormone present in saliva) among children whose working mothers are in jobs with low satisfaction.
Dr Jessop said: "Although there has been a lot of psychosocial work in the past, we are now bringing into play hormonal data.
"So we really have two weapons in our armoury. This provides a very powerful approach to look at the way upbringing and domestic circumstances can affect the way children grow up."
He said larger studies over a longer period of time were needed to determine whether children are stressed by their circumstances and whether introducing more social support would help buffer this.