The Spirit Level is the title of a book written by Richard Wilkinson and Kate Pickett, with the subtitle, Why Equality is Better for Everyone.
The authors have compiled information from around 200 different sets of data, using reputable sources dating from 2002, such as the United Nations, the World Bank, the World Health Organisation and the US Census, to form a bank of evidence for campaigning agains inequality. In their book the authors use the information to create a series of scatter-graphs documenting the prevalence of a vast range of social ills. On almost every index of quality of life, or wellness, or deprivation, there is a correlation between a country's level of economic inequality and bad social outcomes. Almost always, Japan and the Scandinavian countries are at the favourable "low" end, and almost always, the UK, the US and Portugal are at the unfavourable "high" end, with Canada, Australasia and continental European countries in between.
The Spirit Level was published by Penguin in 2010. For those wishing to help reduce inequalilty there is an associated website of The Equality Trust, with downloadable resources.
You can see videos of a lecture by Wilkinson and Pickett at:

This wikipage introduces ways in which the resources can be used in education to discuss the concept of 'just shares'.

As an introduction to the powerful educational messages that can be derived from the data, the following scatter diagram plots life expectancy against national income.
Above all, this diagram illustrates the global range of national incomes. It also suggests that as national incomes rise during economic development so does life expectancy. At first, economic development funds are associated with reduced mortality in the younger age groups. Modern preventive and therapeutic medical care can account for only a minor fraction of the dramatic improve­ments in individual and population health over the past 250 years. Even in the United States, only about five years of the 30-year increase in life expectancy is attributed to preventive or therapeutic medical care. The remainder is attributable primarily to increasing socio-economic development and associated gains in nutrition, public health and sanitation, and living conditions. The five African countries set apart from the general trend in life expectancy, indicated by the cluster contained within the red lines, namely Namibia, South Africa, Equatorial Guinea and Botswana. have lower life expectancy than expected from the average national income. With respect to Botswana, almost all of the country's 1.7 million people are affected by HIV in some way. Without AIDS, life expectancy in Botswana would be 72, according to the U.S. Census Bureau. While life expectancy is rising in most developed countries, it has declined drastically in Botswana since the HIV rampage began. It is now lesss than 40 years In 2005, Botswana received $17 million for prevention activities from the USA. This was 37.7 percent of the country's total funding for prevention, care and treatment. Another $12.3 million — or 27.2 percent — went to care, while treatment activities were allocated $15.9 million — or 35.1 percent. Premature deaths from AIDS is particularly high in the other three countries in the cluster. However, in general, for any given life expectancy the large lateral, left to right, spread in national incomes indicates that there are many factors influencing mortality at all ages.

In this diagram, life expectancy for each country is plotted against the average CO2 emissions for each person in that country. The average CO2 emission per person is indicated by the purple line. Countries marked by red dots all have life expectancies in the region of 70-80 years, but half of them fall below the world average per capita CO2 production. At this high value of life expectancy there is no correlation between life expectancy and CO2 emissions, e.g. people in Australia and the United Arab Emirates emit ten times more CO2 per person than Costa Rica but have the high same life expectancy.

The ecological footprint is a measure of human demand on the Earth's ecosystems. It is a standardized measure of demand for natural capital that may be contrasted with the planet's ecological capacity (biocapacity) to regenerate. It represents the amount of biologically productive land and sea area necessary to supply the resources a human population consumes, and to mitigate associated waste. Using this assessment, it is possible to estimate how much of the Earth (or how many planet Earths) it would take to support humanity if everybody followed a given lifestyle. For 2006, humanity's total ecological footprint was estimated at 1.4 planet Earths – in other words, humanity uses ecological services 1.4 times as fast as Earth can renew them. Every year, this number is recalculated — with a three year lag due to the time it takes for the UN to collect and publish all the underlying statistics.

The Human Development Index (HDI) is a comparative measure of life expectancy, literacy, education and standards of living for countries worldwide. It is a standard means of measuring well-being, especially child welfare. It is used to distinguish whether the country is a developed, a developing or an under-developed country, and also to measure the impact of economic policies on quality of life. There are also HDI for states, cities, villages, etc. by local organizations or companies.

In the following diagram the ecological footprint of different countries is plotted against their human development index. The planet's biocapacity has been divided by the world's population to give a figure representing each person's share if it were to be equally divided. It is marked by the green line. The aim of each country's plans for economic development historically has been to raise biocapacity available per person above this basic level which may be descibed as the level of sustainable demand . The diagram shows that, in this context, the human demand on the environment exceeds the world's biocapacity at a development index of around 0.8. This threshold of world development is marked by the blue line. However, there is one country that has achieved the breakthrough whilst maintaining an ecological footprint that is sustainable. This country is Cuba, and its position is marked by a red dot. It has achieved levels of life expectancy and infant mortality which are almost the same as the oil rich states of the Middle East, and close to that of the USA.

An apparent paradox, Cuba, with a GDP of US$31.72 billion (2004 data, held constant at 2000 prices), outperforms some high and middle income countries in indicators such as infant mortality; under five mortality; life expectancy; adult literacy; and physician to patient ratio. Cuba's CO2 emissions peaked in the mid 1980s but are now rising again. But still the latest figures for 2006 show that Cuba ranked 50th out of 177 countries in the most recent Human Development Index.
In 2009 Cuba ranked 76 in the world CO2 emissions by country agains the USA at 2. Its per capita emissions were 2.65 tonnes per capita, as against 17.67 for the USA

Cuba was omitted from the 2010 HDI due to the absence of current internationally reported data for one of the three required indicators: health, education and income (which are used to calculate the composite HDI value, which in turn determines a country's HDI ranking.) The missing indicator for Cuba was for income. There is no internationally reported figure forCuba's Gross National Income adjusted for Purchasing Power Parity (GNI-PPP): the figure used for all countries for the income component of the HDI, and which is normally provided by the World Bank and/or the International Monetary Fund (IMF). Unofficial estimates of GNI-PPP, were considered unreliable by the statisticians and economists at the Human Development Report Office, and the UN Statistical Commission has advised against the use of such imputed - as opposed to officially reported - figures as human development indicators for HDI calculation purposes.