Babies cry to convey emotion, with the main reasons being hunger, pain, anger or fear, researchers find.
But parents – particularly first time parents – can struggle to identify which is the underlying cause, scientists from the University of Valencia say.
A study in the Spanish Journal of Psychology says that the eye activities and the dynamics of the cry can help determine the reason.
Dr Mariano Chóliz said: “When babies cry because of anger or fear, they keep their eyes open but keep them closed when crying in pain.”
The intensity of the cry gradually reduces if the cause is anger, but increases if it is pain or fear, she said.
Dr Choliz said: “Crying is a baby’s principal means of communicating its negative emotions and in the majority of cases the only way they have to express them.”
She and her term studied 20 babies aged between three and 18 months, looking at the pattern of their crying as well as the reaction of the parents.
Anger and fear were the hardest emotion to discern, but Dr Choliz said: “Although the observers cannot recognise the cause properly, when babies cry because they are in pain, this causes a more intense affective reaction than when they cry because of anger or fear.”
Pain was the most easily recognisable emotion, and this could be because it is a warning of a potentially serious threat to health or survival and thus requires the carer to respond urgently.
When a baby cries, facial muscle activity is characterised by lots of tension in the forehead, eyebrows or lips, opening of the mouth and raised cheeks. The researchers observed different patterns between the three negative emotions.
When angry the majority of babies keep their eyes half-closed, either looking in apparently no direction or in a fixed and prominent manner.
Their mouth is either open or half-open and the intensity of their cry increases progressively.
In the case of fear, the eyes remain open almost all the time. Furthermore, at times the infants have a penetrating look and move their head backwards. Their cry seems to be explosive after a gradual increase in tension.
Lastly, pain manifests as constantly closed eyes and when the eyes do open it is only for a few moments and a distant look is held.
In addition, there is a high level of tension in the eye area and the forehead remains frowned. The cry begins at maximum intensity, starting suddenly and immediately after the stimulus.