When a baby becomes stressed, their body produces and secretes the stress hormone cortisol. When a baby is comforted, cortisol is reabsorbed if that comfort happens soon enough, and the stress response is immediately reduced.
If a baby is not comforted enough, there are high levels of cortisol continuously. This has a neurotoxic effect – in other words, brain cells are actually destroyed by this circulating high level of stress hormone. High levels of cortisol also suppress empathy centres in the brain. Therefore a child who is constantly stressed and not comforted find it difficult to be empathetic with others.