Richard Reeves in The Guardian calls the book "a thorough-going attempt to demonstrate scientifically the benefits of a smaller gap between rich and poor", but also says there are problems with the book's approach. "Drawing a line through a series of data points signals nothing concrete about statistical significance [...] since they do not provide any statistical analyses, this can't be verified."[9] John Kay in The Financial Times says that "The evidence presented in the book is mostly a series of scatter diagrams, with a regression line drawn through them. No data is provided on the estimated equations, or on relevant statistical tests,"[10] although the evidence was available online.[11]
In response to criticisms of the book’s lack of statistical data, its 2010 paperback second edition added an Appendix which included such statistical data as Pearson r coefficients of the correlations between income inequality and the book's ten welfare indicators, for both its 23 rich countries and also for the 50 states of the USA. These revealed that important health indicators such as life expectancy and infant mortality both had r values of their correlation with income inequality of less than 0.5 for the 23 rich countries.
Swedish welfare economist Andreas Bergh thinks that Wilkinson and Pickett mistake correlation for causation, and points to Sweden as an example where increased economic inequality has gone hand in hand with better health and better gender equality.[12] Another Swedish Economist, Danne Nordling points out that while Wilkinson and Pickett rank Japan as the country with the highest income equality, all other sources rank Japan as being in the bottom of the OECD table.[13]