Hearing a crying baby appears to cause women’s bodies, but not men’s, to reflexively prepare to take action, according to the results of a study published in Frontiers in Psychology.

The sound of an infant’s cry has been found to have unique effects on adults. It is an especially difficult sound to ignore, and appears to act directly on the nervous system to prepare the adult to take steps to comfort and protect the baby.

It is generally believed that there are evolutionary reasons for this effect. Because human infants are practically helpless, and because crying is their only means of communication in the earliest and most vulnerable months, it is important for the survival of the species that adults have an instinct to respond to these cries quickly. Because mothers have historically been the primary caregivers of infants, there is some suggestion that this instinct to react to babies’ cries is likely to be stronger in women than in men. However, evidence on this question has been mixed.

A team of scientists led by Irene Messina, of the University of Padua, evaluated sex differences in responses to infant’s cries on the timescale of milliseconds by measuring changes in the electric potential in people’s arm muscles. Ten women and ten men, none of whom were parents, took part in the study. Participants were exposed under laboratory conditions to recordings of infants’ cries played at random intervals.

They were also exposed to control sounds, which were infant cries that had been altered in pitch so that they were not recognizable. At the same time, laboratory equipment connected to the participants’ arms monitored the electric potential in their muscles. Higher electric potential indicates readiness for activity.

Women’s arm muscles showed a spike in electric activity about one tenth of a second after being exposed to a recording of an infant’s cry. Under the same conditions, men’s muscles did not show any sign of change in electric activity. The women’s reactions occurred only in response to a natural-sounding infant’s cry, and not to an altered cry or to a control non-cry sound.

The study authors conclude that infant cries elicit an instinctive caregiving response in women that operates at a very basic and automatic level, even in individuals with no parenting experience. However, they caution that their results do not imply that men lack a similar automatic response to infant distress. Men may respond to different cues, or may be primed to react in different ways. Nevertheless, these results provide support for the view that women may have an evolutionarily-driven instinct to move to care for a baby crying in distress.