Before the Second World War, the Ministry of War confidently predicted what would happen when London was bombed from the air by Nazi planes. There would be, they warned, "a mass outbreak of hysterical neurosis among the civilian population". For every one person injured, there would be dozens who lose their morals or lose the plot. They would howl and they would loot and they would rape. Humans couldn't take it. They would break. They would turn on each other.

The same predictions are made about every disaster – that once the lid of a tightly policed civilization is knocked off for a second, humans will become beasts. But the opposite is the case. It sounds grotesque to say we should see reasons for hope as we watch in real time while the earth is shaken six inches on its axis, tsunamis roar, and nuclear power stations teeter on meltdown. But it is true. From this disaster, we can learn something fundamental about our species. It should guide how the Japanese authorities behave today – and kill off right-wing ideologies based on the belief that humans are inherently selfish tomorrow.

The evidence gathered over centuries of disasters, natural and man-made, is overwhelming. The vast majority of people, when a disaster hits, behave in the aftermath as altruists. They organise spontaneously to save their fellow human beings, to share what they have, and to show kindness. They reveal themselves to be better people than they ever expected. When the social scientist Enrico Quarantelli tried to write a thesis on how people descend into chaos and panic after disasters, he concluded: "My God! I can't find any instances of it." On the contrary, he wrote, in disasters "the social order does not break down... Co-operative rather than selfish behaviour predominates". The Blitz Spirit wasn't unique to London: it is universal.

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On 18 April 1906, San Francisco was levelled by an earthquake. Much of the city collapsed, and the rest began to burn. Anna Amelia Holshouser – a middle-aged journalist – was thrown out of bed, and felt her house collapse around her. She wandered the streets, and found herself sleeping that night in the park. But then the daze wore off, and she did what almost everybody else did: she began to look after the people around her. She knitted tents out of old clothes to house all the children who had lost their parents. She set up a soup kitchen, and the local shop-keepers handed over the goods for free. Hundreds of people gathered there, as they were gathering around similar people across the city. Anna put up a sign that said: "One Touch of Nature Makes the Whole World Kin."

In San Francisco that week, the city's plumbers began – unpaid – to fix the broken pipes, one by one. People organised into committees to put out the fires with buckets and anything they could find. The philosopher William James, who watched, wrote: "Everybody was at work... and the discipline and order were practically perfect." It had been an incredibly divided city, prone to race riots against Chinese immigrants. But not after disaster struck. San Fransicans handed out food and clothes to astonished Chinese people. A young girl called Dorothy Day watched her mother give all her clothes to survivors, and wrote: "While the crisis lasted, people loved each other."

They hated what had happened, but loved what they had become. In her gorgeous book A Paradise Built In Hell: The Extraordinary Communities That Arise In Disaster, Rebecca Solnit shows how this is how almost everybody responds to disaster, across continents and across contexts. This is so cross-cultural – from Haiti to New Zealand to Japan – that it is probably part of an evolved instinct inherent to our species, and it's not hard to see why. We now know that 60,000 years ago, the entire human race was reduced to a single tribe of 2,000 human beings wandering the savannahs of Africa. That was it. That was us. If they – our ancestors – didn't have a strong impulse to look out for each other in a crisis, you wouldn't be reading this now. Yet there are a few examples stubbornly fixed in the popular imagination of people reacting to a natural disaster by becoming primal and vicious. Remember the gangs "marauding" through New Orleans, raping and even cannibalising people in the Super-Dome after Hurricane Katrina? It turns out they didn't exist. Years of journalistic investigations showed them to be racist rumours with no factual basis. Yes, there was some "looting" – which consisted of starving people breaking into closed and abandoned shops for food. Of course human beings can behave atrociously – but the aftermath of a disaster seems to be the time when it is least likely.

This information is essential for knowing how to respond to disasters. There is a fear the Japanese government is withholding information about the dangers of the nuclear meltdown because they don't trust the people to react sensibly and calmly. There is no way of knowing, yet, whether this is true. But understanding this crucial history should guide the government to tell the truth. As Solnit puts it: "If you imagine that the public is a danger, you endanger the public." They are the allies of public safety, not its enemy. After the Three Mile Island meltdown in Pennsylvania in 1979, nearly 150,000 people were evacuated. The government was not in charge. Ordinary people spontaneously co-ordinated it themselves, without panic.

In a disaster, very few people are on-yer-bike individualists grabbing for themselves, and they are regarded as incomprehensible by everybody else. After the 2005 tsunami, the Ayn Rand Institute – set up by the philosopher-queen of the American right – issued an appeal entitled: "US Should Not Give Help to Tsunami Victims." Even the people who every day take this callous view of victims within our own societies – the poor, the homeless, the ill – felt the need to distance themselves from this sociopathy. It's often implied that kindness and generosity are naïve, idealistic fictions that will always be trumped by self-interest and greed. This is at the core of a particular kind of right-wing ideology that has been ascendant for 30 years now. But when the stakes are highest, the opposite is the case. When everything else is stripped away, when the buildings fall and the seas rise, we remember all that really matters is caring for each other.

This raises an obvious question. Can we hold onto this impulse after the disaster passes? Can we spread it? Dorothy Day never forgot how her mother behaved that week in San Francisco – and it inspired her to set up a radical movement to house and empower the poor. Can we do the same?

This is likely to be a century of ecological disasters, since each year we destabilize our climate more, in the face of plain scientific warnings. It's hard to extract any hope from the picture this fact presents us with. But there is some. Alongside this impulse to denial and self-destruction, there is something fundamentally good in us. We are humans. We care about each other. We will – at the most crucial and final moment – sacrifice for each other, like the technicians who are trying to prevent the nuclear plant melting down, knowing this is probably personal suicide. That's something to hold onto. Normally, in northern Japan, the night sky is blocked out by the yellow-orange haze of light pollution. Tonight, huddled together, the people there can see the stars.

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j.hari@independent.co.uk; twitter.com/johannhari101