Usually, "optimism" and "realism" are used to describe two different outlooks on life. But I believe that a realistic appraisal of the human condition compels an optimistic worldview. I am particularly optimistic about the potential for technological innovation to improve the lives of the poorest people in the world. That is why I do the work that I do.

Even so, there is one area of technology and global development where reality has tempered my optimism: the idea that cellphones would revolutionize life in developing countries. A decade ago, many people believed that the proliferation of mobile devices in Africa would mean a short leap to digital empowerment. It didn't. Digital empowerment is a long and ongoing process, and the mere existence of cellular technology does not immediately change how poor people meet their basic needs.

But now, after years of investments, digital empowerment is underway, owing to a confluence of factors, including growing network coverage, more capable devices, and an expanding catalogue of applications. As more people get access to better and cheaper digital technology, an inflection point is eventually reached, at which the benefits of providing digital services like banking and healthcare clearly outweigh the costs. Companies are then willing to make the investments required to build new systems, and customers are able to accept the transition costs of adopting new behaviors.

Consider the example of M-Pesa, Kenya's mobile-banking service that allows people to send money via their cellphones. M-Pesa first needed to invest in many brick-and-mortar stores where subscribers could convert the cash they earn into digital money (and back into cash). This real-world infrastructure will be necessary until economies become completely cashless, which will take decades.

Without omnipresent cash points, M-Pesa would be no more convenient than traditional ways of moving money around. At the same time, it was impossible to persuade retail stores to sign on as cash points unless there were enough M-Pesa subscribers to make it profitable for them.

This kind of bootstrapping is exactly what we had to do at Microsoft in the early years of the personal computer. No one wanted a machine unless there was software, and no one would create software unless there were machines. Microsoft convinced both hardware and software companies to bet on future volume by showing how our platform would change the rules.

There have been many successful small-scale pilot programs using cellphones. But examples of large-scale, self-sustaining programs powered by digital technology, like M-Pesa, are harder to find, because the key pieces have not been put into place to enable the required work to advance beyond the limits of controlled experiments.