Letters

Mortality and distribution of income

BMJ 1998; 316 doi: https://doi.org/10.1136/bmj.316.7144.1611a (Published 23 May 1998) Cite this as: BMJ 1998;316:1611

Societies with narrower income distributions are healthier

  1. Stephen Senn, Professor of pharmaceutical and health statistics
  1. Department of Epidemiology and Public Health, Department of Statistical Science, University College London, London WC1E 6BT
  2. Trafford Centre for Graduate Medical Education and Research, University of Sussex, Brighton BN1 9RY
  3. Nuffield Institute for Health, Leeds LS2 9PL
  4. National Primary Care Research and Development Centre, Centre for Health Economics, University of York, York YO1 5DD

    EDITOR—Gravelle's recent contribution to the debate on the relation between income distribution and health may have been difficult reading for some.1 I offer a simple analogy by way of explanation.

    Imagine two identical fields, both of which are manured with equal quantities of a fertiliser. In one field the fertiliser is spread evenly; in the other some plots receive far more fertiliser than others. At harvest, the yield is considerably higher in the evenly treated field. One possible explanation of this result is that, in the unevenly treated field, plants growing faster and higher in plots receiving more fertiliser have drawn moisture away from those less well treated and have also shaded them from the sun. This is analogous to the relative income hypothesis.

    An alternative explanation is simply that the first kg of fertiliser spread on a given plot produces a greater increase in yield than does the second. The optimal application of fertiliser is therefore to spread it as evenly as possible. This is analogous to the absolute income hypothesis. This second explanation works at the level of individual plants and does not need to consider interactions between plants. In the same way, the absolute income hypothesis for health considers only the direct effect of income on the health of an individual. On the other hand, the relative income hypothesis requires that the incomes of others affect the health of an individual through complex societal mechanisms.

    Gravelle's examination is important. The absolute income hypothesis is a simpler explanation than the relative income hypothesis for the observation that, other things being equal, societies with narrower income distributions are healthier. The more complex hypothesis should not be asserted until the claims of the simpler one have been exhausted. In saying, however, that “studies using population level data.. cannot distinguish …

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