A probe into the chemistry of human relationships says that getting a call from mum can be nearly as effective as a maternal hug for calming down after a tough event.
Researchers measured levels of a stress hormone, cortisol, and also a comforter hormone, oxytocin, among 61 young girls who had to make a presentation in public.
The volunteers, aged seven to 12, were asked to do public speaking and then carry out an oral arithmetic test in front of an audience, according to the unusual experiment, reported on Wednesday by the British journal Proceedings of the Royal Society B.
Immediately afterward the event, a third of the girls were physically comforted by their mother; another third received a phone call from mum but did not see or touch her; and the remaining third received no support but watched a neutral film for 75 minutes.
As expected, cortisol levels, measured in saliva, soared as the youngsters became stressed by having to address the public.
But within 30 minutes of the event, cortisol concentrations returned to normal among the children who experienced direct physical contact with their mother.
Among the speech-only group, it took somewhat longer -- about an hour -- for cortisol levels to subside to normal. But among the no-contact group, levels were still more than a third above normal at the one-hour mark.
Similarly, oxytocin concentrations peaked highest among girls who were hugged, followed by girls who were given vocal support but no physical comfort. The surge was still prominent an hour afterwards.
But oxytocin levels remained very low and flat among the "no contact" group of girls who received neither physical nor vocal support.
The findings raise intriguing questions about human evolution, say the researchers led by Leslie Seltzer of the University of Wisconsin at Madison.
Oxytocin is famous as the "cuddle" hormone, a feel-good, trust-making biochemical found only in mammals.
Past research has found that the hormone is released on physical contact, helping to cement attachment between parents and offspring and between couples.
The new experiment confirms for the first time that this powerful hormone can also be triggered by words.
"Our results suggest that vocalisations may be as important as touch to the neuro-endocrine regulation of social bonding in our species," says the paper.
"(...) Vocal cues may be a viable alternative to physical contact for servicing human relationships."
Speech came before oxytocin, and not the other way round, say the authors.
The anatomical apparatus necessary for vocal cues "is at least" 400 million years old, whereas the mechanism for producing oxytocin probably evolved some 200 million years later.
The scientists chose girls for their experiment as they reasoned daughters, rather than sons, would be more accepting to warm physical touch and verbal contact with their mothers.
They also asked pre-pubescent children to carry out the experiment because of the risk that urine samples, used to measure the oxytocin, could be distorted by menstrual hormones.