They focus on the way humans co-operate with each other in many circumstances – including when hundreds of us work for a single business, which competes with other big businesses - and argue that we co-operate not only for self-interested reasons, but also because we are genuinely concerned about the wellbeing of others.

We try to uphold “social norms” of acceptable behaviour, and value behaving ethically for its own sake. For the same reasons, we punish those who exploit the co-operative behaviour of others.

“Contributing to the success of a joint project for the benefit of [your] group, even at a personal cost, evokes feelings of satisfaction, pride, even elation,” they say. “Failing to do so is often a source of shame or guilt,” they say.

We came to have these “moral sentiments”, in Smith’s words, because our ancestors lived in environments, both natural and as constructed by humans, in which groups of individuals who were predisposed to co-operate and uphold ethical norms tended to survive and expand relative to other groups, thereby allowing these “pro-social” motivations to proliferate.

So they explain our motivations for caring about the wellbeing of others: we do it because it makes us feel good. But they also explain the distant evolutionary origins of our disposition to co-operate and its perpetuation to the present day.

Co-operation – engaging with others in a mutually beneficial activity - was part of the behaviour of homo sapiens when we were still living on the African savannah. We formed bands to make us more successful in hunting big animals.

But though co-operation is common in many species, human co-operation is exceptional in that it extends beyond our close relatives – whom we look after in obedience to our evolutionary urge to replicate our species – to include even total strangers. And we co-operate on a much larger scale than other species except the social insects, such as ants and bees.

We co-operate in political and military objectives as well as more prosaic everyday activities: collaboration among the employees in a firm, exchanges between buyers and sellers, and the maintenance of local amenities among neighbours.

So, though they don’t see it in these terms, economists focus on a form of co-operation that involves “reciprocal altruism”. Buyers benefit sellers; sellers benefit buyers.

But human co-operation goes much further, in that it takes place in much larger groups and in circumstances that are unlikely to be repeated. Why do people tip while passing through a country town? In my own town I have reason to care about my reputation. But if I’m in your town, why does it not occur to me to cheat you in some way?

Much experimental and other evidence shows that people gain pleasure from co-operating, or feel morally obliged to. On the other hand, people enjoy punishing those who exploit the co-operation of others, or feel morally obligated to do so.

“Free-riders”, as economists call them, frequently feel guilty and, if they are sanctioned by others, they may feel ashamed.

We may have started out co-operating to hunt wild animals and mind other people’s children, but today we co-operate to enjoy the benefits of “the division of labour” (we each specialise in something we’re good at), of market exchange and the pursuit of economies of scale (in irrigation, factories, information networks) and even warfare.

And we made all this work better by inventing governments capable of enforcing the rights to property and providing incentives for the self-interested to contribute to common projects.