On one level, the approach of natural scientists is correct. There must be a physical limit to the number of humans that can be supported by the earth. In practice, however, the approach of social scientists is the most relevant. Human beings live in society and for millennia, the constraints on population growth are not physical but social and political. Famines, as Jean Drèze and Amartya Sen demonstrated, are usually the result of political failures, not natural causes.
It would be nice if we could understand human society as a natural system, but unfortunately we cannot. In modern economies, people make decisions – about everything from what to buy to how many children to have – based on economic and social incentives, not physical needs. And while physical needs and the earth’s ability to meet them may be fixed, social needs and the economy’s ability to meet them are not.
In the physical domain, the needs do not change: we need the same number of calories today to survive as our ancestors did 500 generations ago. In the social sphere, however, what is valued today is often worthless tomorrow, and people’s behavior changes accordingly. Just ask BlackBerry or Nokia management. Conversely, things that we would not even have imagined today could be considered essential in five years. Just ask Mark Zuckerberg or, more prosaically, whoever invented the chain of cafes on the high street.