By the 17th century, Islam had been the religion of the world’s primary military, commercial and cultural powers (in various incarnations) for a millennium. Thereafter, the temporal and economic power of Islamic states has been in steep relative decline in relation to their Christian counterparts, but adherence has proved remarkably resilient. Currently, approximately a quarter of the world’s population is Muslim, and on current projections it will overtake Christianity in total adherents by 2050. This is in part a factor of relative economic failure: Muslim countries have not experienced the depression in birth rates commonly associated with first world affluence. However, with a heavy concentration of adherents in the arid subtropics, they have enjoyed huge jumps in life expectancy through the medical revolution and improvements in sanitation and hygiene. In addition, Muslims have established buoyant migrant populations in the west, while Christian minorities in Islamic states are often in decline.