Deborah paces the floor in the hospital waiting room. Her daughter is undergoing her third session of chemotherapy. Deborah imagines how the freezing poison — however necessary it may be — is spreading through Catherine’s veins. It’s as if she, too, can feel the nausea moving through her daughter, as tension grips her stomach. She would give anything to be ill in her daughter’s place.

Robbie stares at his television set: refugees are fleeing war in their country. They have been on the road for days, without food or water. They carry bags tied with string. A father with haunted eyes holds his dead child. The camera lingers on his arms uselessly clutching the little boy close to his chest. Robbie cannot sit still. He is a doctor. He wants to do something. Days later, he leaves with Médecins Sans Frontières.

When we suffer, our body takes action in order to cope. This is the famous fight-or-flight reaction. But where does this wish to take on the suffering of someone we deeply care about come from — this powerful urge to do everything within our power to provide relief, as if we ourselves were suffering?

In Professor Uta Frith’s cerebral imaging laboratory at University College London, women agreed to enter a magnetic resonance scanner while their husbands received electric shocks. Each woman was warned a few seconds in advance that the shock was about to be given, and each could see her husband’s hand in a mirror as it flinched with pain. The pain they felt on seeing the man they loved suffer was discernable on their faces, but, more interestingly, the regions of the brain that were activated were virtually the same as if they themselves had received an electric shock.

For these women, linked by love to their husbands, the pain had become their own; the barrier separating ‘me’ from ‘you’ had been broken.

‘Ya pihi irakema (I am contaminated by you),’ say the Yanomami Indians of the Brazilian rainforest when they are in love. This means, ‘Something from you has entered me, and is now living in me.’ The US philosopher Susanne Langer says that, under the influence of love, the ‘membrane of individuality’ becomes porous.

Of course, some people are more sensitive than others to this empathy. Women tend to have a greater capacity for empathy than men. And among women — and men — there are significant differences. But our brain’s automatic reaction is the very foundation of our humanity. It underpins our capacity for connection with others.

What makes mammals different from all other animals is not just that mothers breastfeed, but that the emotional regions of the brain produce a link between parents (particularly mothers) and their offspring. The anterior cingulate cortex (seen in action when Frith’s subjects saw their husbands suffer) evolved so that the cries of a child separated from its mother would be unbearable to her. This way, constant contact with an adult — vital to the life of small, fragile mammals — is ensured.

Beyond this evolutionary link, our capacity for compassion is the basis of the doctor’s vocation, the volunteer’s urge to help and our desire for harmony within our community. It is also the foundation of ethics, as held by Spinoza, a seventeenth-century philosopher. In our capacity to feel another’s emotions, he saw the origin of morality: if he suffers, I suffer, so I must try to stop his suffering. This bond that links us to the emotions of those around us is inscribed deep within our brain. It is what makes us human: individual and connected, sensitive and responsible.