 When the social variable is income, there is an important further definitional dispute to consider. Income appears to have a significant effect on life expectancy, even controlling for education (Backlund, Sorlie, &amp; Johnson 1999). However, there is an on-going debate about which definition of “income” is adequate to capture the contribution individual income makes to individual life expectancy (see the sample of articles collected in Kawachi, Kennedy, &amp; Wilkinson 1999). According to the absolute income hypothesis, the contribution income makes to individual life expectancy is entirely a function of the individual’s non-comparative income. By contrast, the relative income hypothesis holds, roughly, that an individual’s life expectancy is also a function of the relative level of her income—that is, its level compared to others’ income in her society—and not simply of its non-comparative level (Wilkinson 1996). To make this second hypothesis precise requires one, among other things, to specify the reference group to which the individual’s income is compared and also the nature of the comparison (for examples, see Deaton 2003). For an overview of this debate and the state of evidence, see Sreenivasan (2009a) and Pickett and Wilkinson (2015).