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Hearing a baby cry can trigger a range of responses in adults, such as sympathy, anxiety and a strong urge to help. However, new research suggests that a deeper physical reaction is also occurring. A baby's cry, particularly if it is in pain or distress, makes our faces physically warmer.

Since they can't speak yet, babies cry to communicate their needs, whether they're in pain or want some attention. When a baby is in distress, they forcefully contract their ribcage, which produces high-pressure air that causes their vocal cords to vibrate chaotically. This produces complex disharmonious sounds known as nonlinear phenomena (NLP).

To study how adults respond to crying babies, scientists played 23 different recordings to 41 men and women with little to no experience with young infants. At the same time, a thermal infrared imaging camera measured subtle changes to their facial temperatures. A rise in temperature in this part of the body is governed by the autonomic nervous system, a network of nerves that controls unconscious processes such as breathing and digestion. After each cry, the participants rated whether the baby was in discomfort or in pain.

The study found that adults' facial temperatures change when they hear a baby cry, a clear sign that the autonomic nervous system has been activated. This suggests that people unconsciously pick up on acoustic features in a baby's cry. The higher the level of NLP (meaning a baby is in more pain or distress), the stronger and more in sync the listener's facial temperature became. In other words, as the cry grew louder, a person's face grew warmer. This physiological reaction was the same for both men and women.

"Our results demonstrate that the level of nonlinear phenomena (NLP) in a cry modulates the temporal dynamics of the facial thermal response in listeners, independent of their sex," wrote the scientists in their paper in the Journal of the Royal Society Interface.

While the results are fascinating, the researchers note that their study has some limitations. They only worked with inexperienced adults, and therefore, the results may differ from those with parents and seasoned caregivers. Also, the team used natural cries and not ones they could manipulate in a laboratory setting, so they could not determine which specific NLP elicited which type of response.

Future research may give us more insights into how adults respond to baby cries. This could be useful in a number of areas. For example, to screen caregivers by seeing how they react naturally to infants' wailing and to help parents who may be struggling to understand what their baby is trying to communicate.