Imagine a green forest that separates two tribes of shepherds, each with distinctive beliefs and moral codes. One hot, dry summer, the forest burns down, and after the rains come, a lush new pasture grows between the tribal lands. Do they share — or fight?

“This is the problem of modern morality,” said psychologist Joshua Greene. “It’s not good versus evil. It’s not about me versus us. It’s about us versus them. It’s about how different groups are going to cooperate, and how they’re going to get along in the new pasture of the modern world.”

Morality helps societies deal with the tension between self-interest and the common good, which is the fundamental problem of social existence, Greene said in a conversation with fellow Harvard scholar Steven Pinker, Johnstone Family Professor of Psychology, and Princeton philosopher Peter Singer at Spangler Auditorium. The panel was part of an Oct. 21 symposium sponsored by the Edmond J. Safra Center for Ethics, Harvard Business School, and the Center for Public Leadership at HKS.

‘Deep pragmatism’ as a moral engine

For contemporary conflicts, there might be need for a “meta-morality” that could help groups with different traditions and moral codes resolve disputes, said Greene, who directs the Moral Cognition Lab at Harvard.

“The job of morality is to adjudicate among the competing interests and values of individuals,” he said. “The modern problem calls for a meta-morality to adjudicate among the competing interests and values of different tribes.”

Singer returned to the individual, saying that being a decent person involves doing the most good. “We’re ethical if we help others,” he said. “But to change the world, we need to do the most good, not just do good.”

One path is effective altruism, a movement inspired by Singer’s work, which calls for donations that help hundreds of people rather than a few. Among such causes, he said, are global poverty and poverty-related preventable diseases around the world.

In his 2009 book, “The Life You Can Save,” Singer argued that affluent nations have a moral obligation to help end poverty in the developing world. According to the World Bank, extreme poverty has fallen to under 10 percent of the global population for the first time in modern history, but Singer said much more could be done.

In the end, the goal should be to reduce suffering and maximize happiness, he said.

“There is an increasing interest in measuring national progress not through GDP but through GNH — gross national happiness,” said Singer. “It is an important initiative to promote, and if we can find ways of assessing whether policies make people happy or not, then in addition to reducing suffering, we should be looking into this.”

Pinker made the case for moral progress. Despite devastating conflict in the Middle East and episodes of violence in the United States and around the world, “we’re living in the most peaceful times in our species’ existence,” he said.

As evidence of moral progress, Pinker pointed to declines in wars between nations, global homicide rates, extreme poverty, and child mortality. He also cited improved literacy rates and education, and the impact of a rise in wealth.

“Money absolutely can buy happiness,” he said to laughter in the room. “The wealthier people are, on average, the happier they are.”

Compared to a century ago, the world is much better off, Pinker stressed, asking the audience to share his optimism.

“There is an enormous amount of irrational pessimism,” he said. “Let’s stop complaining so much and figure out how to make it better, still.”