In the 20th century, increased production was a fairly successful response to absolute poverty, which had been the lot of most of the human race for most of history. But in the 21st century, that path has come to a dead end, as the planet reaches its resource limits. A more equal distribution of wealth needs to return to the centre of development theory.
There are many types of poverty but the one most people associate with international development is the absolute kind. If you are likely to die in childbirth, or your kids can't go to school because they have to work, or you don't eat enough food every day to keep your body functioning properly, then you are poor in this absolute sense.
Absolute poverty declined faster in the last century than it has at any time in human history. It has been eradicated in the 60 or so high-income countries, and reduced to pockets in most of the almost 50 upper middle-income countries.
Crucially, it has reduced in most poor countries too. After the lost decade of the 1980s, the recent reports on the millennium development goals show that the last couple of decades in particular have shown great gains in human development (although the poorest, as ever, aren't doing as well as the richest).
But these gains are under threat. To understand the future of development we have to look briefly at its history.
In the early days of economics, when Adam Smith, David Ricardo and Thomas Robert Malthus were laying down the foundations of a new social science, the study of inequality (ie the distribution of wealth) was paramount. Economic theory suggested that working men and women would live on or around the poverty line as a consequence of the accumulation of capital and the need to keep wages low. Redistribution, however unlikely (Karl Marx thought it would require revolution), was the only way to address this inevitable and widespread poverty.
But when I studied economics a few years ago, I had to ask the lecturers to add a session on inequality to the curriculum. One of the broader-minded professors agreed, but they still wouldn't include a question on it in the exam. It had simply fallen off the curriculum of modern economists.
Why? Because the early economists' predictions turned out to be wrong on this. Capitalism in industrialising societies led to significant gains in the living standards of the average person. Working people started to worry less about equality when it became clear just how much their lives were improving. So economics – and development – became about how to grow production, and equality became a forgotten area of study.
The political struggles for more just societies played their part in ensuring that the fruits of wealth were better shared, and life would have been even better for the poor had struggles for more equality been much more successful.
But, ultimately, increased production in the last century was an adequate answer to the problem of absolute poverty.
So why can't we just keep going on with more of the same in this century? Why can't the world's poorest countries follow the example of the already industrialised?
The answer is simple. There is physically not enough to go around. We have reached the limits of our planet's resources. In a world with limitless copper, coal and iron, there is, in theory, enough for everyone. This is the world inhabited by rich-world economists and politicians who are used to scouring the planet for all their country's need.
But in the real world, there is a limited amount. What the rich countries take, the poor cannot have. Inequality, both within and between countries is on the rise. Fears about food production and erratic climate change are perhaps the two areas where the world is most dangerously off-balance.
As resources run thin, more production is no longer an option to respond to persistent poverty. Redistribution has to come into play.
Another thing the early economists got wrong was the fraught issue of population. Malthus believed that as wealth increased, so would population. But the opposite appears to be true. Along with access to contraception, simply becoming better off appears to have encouraged parents to have fewer children rather than more.
This is good news, and it is another urgent argument for better distribution of wealth. Wealthier communities will see population decreases, which will in turn make living on our planet more sustainable.
Redistribution is the hardest of political ambitions, because very few people are willing to see their children's standard of living reduce below theirs. In the past, violent revolution has been the way to achieve serious redistribution. This time around, we need a revolution in our expectations to place equality at the heart of a new development paradigm. If it is not swift and wise, then it may be violent. It will certainly require massive and sustained political pressure.
So we need to find a way of making reduced consumption aspirational. We need to convince each other that living with fewer things is good for us.
We need a new development theory for a new century. There have always been many reasons to fight for equality. Ending absolute poverty is once again definitively one of them.