Treatment with the "cuddle" hormone oxytocin can improve the social abilities of some children with autism, research has shown.
Scientists used a nasal spray to administer the hormone or an inactive "dummy" placebo to 32 children with autism twice daily for a month.
Children who had the lowest natural levels of oxytocin responded most to the hormone and showed the greatest improvements in social behaviour.
They found it easier to engage socially with other children than they did before the treatment. But oxytocin did not reduce repetitive behaviour, another trait of children with autism, or anxiety.
Lead researcher Dr Karen Parker, from Stanford University in the US, said: "Our results suggest that some children with autism will benefit from oxytocin treatment more than others, and that blood oxytocin levels might be a biological sign that will allow us to predict if a child will respond maximally or not."
Oxytocin is known to promote social bonding and has been nicknamed the "love" or "cuddle" hormone.
Levels of the hormone are raised during pregnancy and when a mother has contact with her baby. High levels of the "love hormone" are also seen in couples in the first six months of a romantic relationship.
Study co-author Professor Antonio Hardan, also from Stanford University, said: "Hopefully, this is a first step to identifying the characteristics of people with autism who respond to specific treatments.
"Because of the heterogeneity (wide ranging nature) of the disorder, we need to start doing clinical trials not to see if there will be a response, but more to see who will respond to possible treatments."
The findings are reported in the journal Proceedings of the National Academy of Sciences.