The principal rationale for prioritising help to the world's poorest is how little money it could take to improve their welfare.
In childhood, we are told stories of how rich people are bad and poor people are good. It is something of a problem, given that, globally, we are the richest generation in human history: Mediaeval royalty had poor healthcare and life expectancy - and no Internet. However, we can take some comfort from the fact that, by its end, we can expect to have been among the poorest generations of the still new millennium. Even Bill Gates will have many deprivations compared to some future generations.
On the other hand, as we grow up and put away childish things, we are increasingly told that the poor are feckless, tax is theft and that the rich work hard for their money - as if the poor do not.
If it sounds like we are being hit by propaganda, that is hardly surprising given how recently the Cold War ended. Of course, the propaganda is also frequently self-serving: few people argue that they should be paid less for what they do. A more reasonable assessment is the one the Irish writer and critic Mary Colum expressed to Ernest Hemingway: "the only difference between the rich and other people is that the rich have more money." In the same way, the powerful are like the weak just with more power.
Who is rich and who is poor? Apparently most millionaires do not see themselves as rich. Compared with the bottom billion, however, the world's top 3 billion are very wealthy, to say nothing of the top 1 billion. However, many of that top 1 billion are currently unemployed, depressed or facing serious health difficulties. Many of the bottom 1 billion are fairly happy. Both groups contain what would commonly be regarded as nice people and nasty people. Both groups contain men, women and children who are suffering and who could be helped.
Many of the biggest global health problems are common to every country: heart disease, cancer, depression, obesity, smoking, road accidents. These problems often relatively neglected in lower-income countries despite their increasing life expectancies and growing problems with tobacco marketing and obesity, for example.
Nevertheless, rich and poor countries and populations do not experience the same burdens of disease and disability. When it comes to children, the differences are stark. Millions of children under five die every year from poverty. Every 20 seconds a child dies from diarrhoea. UNICEF estimates that 165 million children are stunted due to malnutrition. Citizens of the rich world enjoy democracy, human rights, public services, safety nets and peace in far greater measure than the bottom billion.