On the day India surpasses China as the world’s most populous nation, the change for both countries will be psychological and symbolic. China will still be the greater economic power, the one challenging the United States for full superpower status, but it will no longer be able to call itself the largest nation by sheer numbers. On present trends, the population gap will continue to widen rapidly, but what that will mean for their relative clout in the world will be decided by a host of other factors such as investment and governance. Demographics are not destiny. However, the moment...