Unequal societies like Britain and the USA perform worse on almost all measures of quality of life such as life expectancy, obesity levels, crime rates, literacy, even the quantity of recycled trash. (The model also works within the US: with rich and unequal states such as California underperforming compared to the less productive Mormon paradise that is Utah.)
As David Runciman notes in his LRB review of this book, the evidence presented doesn't always wholeheartedly support the argument. The data on infant mortality presents us with the best fit — so we see that children from the highest social class in the UK are slightly more likely to die than children from the lowest in Sweden — but elsewhere it's harder to insist that everyone is made worse off by inequality.
In the US the reigning perspective is that it hardly matters if the bottom 20% are completely screwed just as long as the other 80% are able to delude themselves into thinking that they have it better than everyone else in the affluent world.
As Runciman notes, it thus seems politically reasonable to some to argue that the poorest group in unequal societies are in a sense cut adrift (and can be treated as such) and to ignore average statistics of wellbeing precisely because they have been brought down for the society as a whole by the underclasses.
"This is why the difference between ‘almost everyone’ and ‘everyone on average’ matters so much: politics. If it is almost everyone who would benefit from a more equal society, then this is an encouragement to solidarity across social boundaries, so that joint action to remedy the problem might be possible. But if it is everyone on average, then this can go along with an absence of solidarity and the hardening of divisions, because the disadvantages may be so unequally distributed."
"The practical political difficulties of bridging the gap between these two positions are clear from Obama’s recent speech on healthcare reform. He wants to be able to say to the American public that everyone will be better off under a reformed system – indeed, in an earlier, far wonkier speech he made to the American Medical Association in June he sounded pretty much like the authors of The Spirit Level: ‘Today, we are spending over $2 trillion a year on healthcare – almost 50 per cent more per person than the next most costly nation. And yet . . . for all this spending, more of our citizens are uninsured; the quality of our care is often lower; and we aren’t any healthier. In fact, citizens in some countries that spend substantially less than we do are actually living longer than we do.’ But he knows that most Americans think that the problems of their system are heavily concentrated at the bottom end, among the uninsured. So, as the politics got more fractious over the summer, this is where he directed his argument: not at the idea that the present system leaves almost everyone worse off, but at the thought that almost anyone could suddenly fall through the hole at the bottom. ‘Everyone understands the extraordinary hardships that are placed on the uninsured,’ he said to Congress in September. ‘We are the only wealthy nation that allows such hardship for millions of its people. There are now more than thirty million American citizens who cannot get coverage. In just a two-year period, one in every three Americans goes without healthcare coverage at some point. And every day, 14,000 Americans lose their coverage. In other words, it can happen to anyone.’"
Another problem for the authors of The Spirit Level is the question of policy. Many of the more equal societies in the developed world (such as Japan and Sweden) became that way as a result of peculiar historical circumstances and not (just) because a progressively-minded government decided to implement piecemeal policy changes aimed at stealthily reducing inequality.
Yet where inequality can be shown to be bad for 'almost everyone' and not just certain socio-economic groups, the data should be able to inform a set of policies geared to take note of the common underlying factors behind a whole range of different social ills that beset otherwise wealthy societies.
It would also be very interesting to see how the data plays out in the developing world and to consider for example, whether 'almost everyone' in Latin America would have access to better education if the inequalities of wealth here could be reduced. A straightforward one-on-one comparison with Asia would seem to indicate that this is indeed the case.