Author's reply to Hanretty and Ford.

I thank Hanretty and Ford for their comment. They make an interesting point, but I think that this has been over-made recently. The term “size of the denominator population” explains that some population groups are bigger than others and that this matters greatly. The map published with the article shows the number of voters polled in each region, adjusted to remove sampling bias. The proportion who voted Leave in each area is shown as an arc, and these are summed to illustrate how the absolute numbers of Leave voters were higher in the south. Geographers usually define the UK north-south divide by splitting the Midlands in half. Maps can help clarify the importance of differing relative risks, especially with symbols or projections that account for the differing size of the denominator populations. On 28 November I’m giving the Political Studies Association lecture, “Another world is inevitable: mappingUKgeneral elections—past, present and future.” I’ll try to explain more then and to include some examples of mapping in medical journals. Future political analysis could benefit from knowing that not just the highest relative risks matter but the overall spread of an affliction (or its causes). Furthermore, connections between politics and health are often not well known, and I’ve speculated on some regarding Brexit. In the five years before the Brexit vote the self reported health of the UK population became much worse, year on year. This mostly affected older people. Such rapid deterioration in people’s health may well have affected how those who got to vote on 23 June felt about their lives. As Rudolf Ludwig Carl Virchow put it simply many years ago, “Medicine is a social science, and politics is nothing but medicine writ large.”