Commentary: Money and models: double-edged swords

Being richer rather than poorer is well known to be associated with a longer and healthier life. It is a small leap from this long-standing observation to the proposition that giving money to people could offer them this same advantage. But evaluation of this simple premise has been hampered by the nonrandom fashion in which wealth is typically distributed. Apart from lotteries, people rarely receive extra doses of wealth at random, and lotteries are problematic because those at risk of winning are the volunteer subset of the population who bought tickets, and therefore have uncertain generalizability. Moreover, if money is a treatment, then one also has to be specific about the timing and magnitude of the dosage. Whereas one 75-mg aspirin pill every day might reduce the risk of a heart attack, for example, 180 pills taken at once every 6 months would have a very different effect, despite being the same annual amount. Bruckner et al. are interested in exactly these two important questions: first, can one identify a causal effect of increased wealth on mortality through some fortuitous exogeneity, and secondly, could a large intermittent dose, such as a lottery win, actually have a harmful effect? The working hypothesis is that people might use a windfall to treat themselves to harmful products, such as intoxicants and fast cars, thereby elevating their risk of adverse events. The authors therefore turn their attention to accidental deaths, including those due to motor vehicles, poisoning and overdose. As a ‘natural experiment’ for the quasi-random assignment of income windfalls, they capitalize on the 1995 placement of a casino among a rural population of Cherokee Indians in Western North Carolina. The casino operator was obligated to make ongoing payments to the enrolled members of the tribe as a percentage of profits, and this amounted to something in excess of US $5000 per person each year. These payments raised the incomes of tribal members considerably, but they did so through one or two large instalments each year, and the authors speculate that this acute dosing of wealth supplements could prove to be detrimental. Bruckner et al. examine whether the months in which payments were received demonstrate excess deaths from accidents among the 57 000 residents of three counties in which the tribal members live. They compare the pattern of payments across the 204 months from January 1990 to December 2006 with the monthly incidence of accidental death in the Native American population up to the age of 55 years in these counties. Randomization is attractive as a means of identifying a causal effect because the potential outcome will be independent of the receipt of exposure, and therefore unconfounded in expectation. A so-called ‘natural experiment’ simply involves an independency that arises through some inadvertent mechanism, as opposed to intentional randomization designed and implemented by researchers. Unfortunately, the treatment schedule in this instance was not at all random. Annual payments commenced on 1 December 1995. Starting in 1998, this annual lump sum was divided into a two semi-annual sums, distributed every 1 December and 1 June. Therefore, Cherokee adults received cheques every December since the opening of the casino. After 3 years of annual sums, the payments were split and arrived the first day of every December and June. There were therefore 21 out of 204 months in which payments were received (12 Decembers and 9 Junes). With respect to the outcome measure, 1995 may have been a ‘random’ year in which to initiate the payment regimen: but, in every subsequent year, the monthly schedule was fixed with absolute certainty. The obvious problem here is that if excess accidental deaths occur in December and June for other reasons, the analytic design assures that these excesses will be attributed to the delivery of cheques from the casino. Unfortunately, the authors employ no control group (e.g. non-Cherokee in the same counties) with which Published by Oxford University Press on behalf of the International Epidemiological Association

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