[4] The book argues that there are "pernicious effects that inequality has on societies: eroding trust, increasing anxiety and illness, (and) encouraging excessive consumption".

In a review for Nature, Michael Sargent said that "In their new book, epidemiologists Richard Wilkinson and Kate Pickett extend this idea" (of the harm caused by status differences) "with a far-reaching analysis of the social consequences of income inequality.

Using statistics from reputable independent sources, they compare indices of health and social development in 23 of the world's richest nations and in the individual US states.

Richard Wilkinson responded to the review in a letter, claiming that "while pointing out that we do not have evidence on the fraction of one percent who are very rich, we show that people at all other levels of the social hierarchy do better in more equal societies".

[11] Boyd Tonkin, writing in The Independent, described it as "an intellectual flagship of post-crisis compassion, this reader-friendly fusion of number-crunching and moral uplift has helped steer a debate about the route to a kinder, fairer nation.

[16] John Kay in The Financial Times said that "the evidence presented in the book is mostly a series of scatter diagrams, with a regression line drawn through them.

He concludes that both the psycho-social effects described in The Spirit Level and the impact of diminishing returns to income likely both contribute to explaining the observed correlations, and questions why Wilkinson and Pickett neglect to mention the latter explanation in their book.

[26] In 2010, Tino Sanandaji and others wrote an article for the Wall Street Journal in which they said, "when we attempted to duplicate their findings with data from the U.N. and the Organisation for Economic Co-operation and Development (OECD), we found no such correlation".

'[28] Peter Robert Saunders, Professor Emeritus of Sociology at Sussex University, published a report for the think tank Policy Exchange questioning the statistics in The Spirit Level.

[36] In response to criticism of the book, Wilkinson and Pickett posted a note on the Equality Trust website which stated: "Almost all of the research presented and synthesised in The Spirit Level had previously been peer-reviewed, and is fully referenced therein.

In order to distinguish between well founded criticism and unsubstantiated claims made for political purposes, all future debate should take place in peer-reviewed publications.

[40] David Cameron referred to the thesis of "The Spirit Level" in his 2010 Hugo Young Lecture, arguing "research by Richard Wilkson and Katie [sic] Pickett has shown that among the richest countries, it's the more unequal ones that do worse according to almost every quality of life indicator".

[46] Think tank Class in association with The Equality Trust and My Fair London published a booklet drawing on The Spirit Level as well as presenting other essential information about income inequality.

[47] In 2010, Richard Wilkinson was appointed the chair of Islington's Fairness Commission to examine ways of reducing income inequality in the London borough.

[48] In July 2024, the authors noted inequality had worsened since the publication of their book 15 years ago, and presented an updated report on their findings to the then incomming UK Labour government.