It is tempting, as the 2015 target date for the Millennium Development Goals (MDGs) creeps relentlessly closer, to take stock of our chances of actually meeting them. Most experts paint a mixed picture. The consensus is that while there has been a good deal of progress in many areas, especially in Asia, Africa’s outlook has worsened considerably. The Four Horsemen of the Apocalypse, it seems, are still rampaging on the dark continent. The UN’s 2008 MDG Report found that ‘little progress was made in reducing extreme poverty in sub-Saharan Africa.’ Its troubled history, challenging geophysical conditions and countless governance problems seem to conspire to push Africans into ever more hopeless poverty. Right?

Does hostility by outsiders generate generosity and solidarity towards fellow group members? This question sheds light on a puzzle that has preoccupied social scientists since Darwin’s time. How can pro-social behaviour – including a desire for sharing and helping others – be evolutionarily stable?