For the head of the World Bank Group to quit unexpectedly would have been big news under any circumstances. But the reason outgoing president Jim Yong Kim gave for his resignation on Monday is even more revealing.

Mr Kim said he was leaving the world's most influential development and infrastructure-building agency to join a private-sector infrastructure investment fund because he believed "this is the path through which I will be able to make the largest impact on major global issues like climate change and the infrastructure deficit in emerging markets".