We’re sometimes asked whether life-saving interventions (such as insecticide-treated net distribution) risk leading to overpopulation. A common response is that (as argued by Hans Rosling of Gapminder and the most recent Gates Foundation annual letter, among others) the reverse dynamic holds: population growth tends to slow as child mortality declines, possibly because parents feel less need to have multiple children in order to hedge against the risk of death.

Empirically, it appears generally true that lower child mortality tends to correlate with slower population growth. However, one might ask whether this relationship is causal (i.e., falling mortality leads to falling fertility, for reasons outlined in the previous paragraph) or merely correlational (i.e., falling mortality generally accompanies a host of factors related to general economic development, and including falling population growth). This question is important when assessing the impact of a life-saving intervention in isolation: it’s possible that health interventions, unaccompanied by improvements on other dimensions, do in fact lead to higher populations.

We’ve long wondered about this question, and we recently commissioned David Roodman – whom we have been impressed with in the past – to examine the rather extensive and complex literature on it. David has recently posted a draft of his writeup, and is seeking feedback.

We haven’t yet fully vetted the writeup, but it appears to me to give a deep and balanced examination of the available literature, along with interesting discussion of the general challenges of isolating causality in cross-country studies. Its conclusion:

I think the best interpretation of the available evidence is that the impact of life-saving interventions on fertility and population growth varies by context, above all with total fertility, and is rarely greater than 1:1 [rarely enough for the fertility decline to exceed the mortality decline in terms of effect on population size]. In places where lifetime births/woman has converging to 2 or lower, family size is largely a conscious choice, made with an ideal family size in mind, and achieved in part by access to modern contraception. In those contexts, saving one child’s life should lead parents to avert a birth they would otherwise have. The impact of mortality drops on fertility will be nearly 1:1, so population growth will hardly change.

Overall, it appears that life-saving interventions unaccompanied by other improvements, where access to contraception is weak, are likely to lead to some acceleration of population growth. With that said, we wish to note the following:

No intervention takes place in isolation, and we expect population growth to slow in the future in most low-income areas as poverty falls.

Acceleration of population growth should not necessarily be associated with overpopulation and its connotations of a net decline in standards of living.

GiveWell’s current top charities are all focused on improving living standards rather than averting deaths, and Against Malaria Foundation (a former and potentially future top-rated charity) aims to avert deaths and bring about other benefits by reducing the burden of malaria.

We encourage our readers to check out the writeup and send in their thoughts. It’s a thorough review of the question and a generally interesting read.

More on why we commissioned this writeup

We have previously made some preliminary attempts to review the literature on the connection between mortality and fertility. However, we’ve found this literature to be (a) challenging to synthesize, as most of the key papers use relatively complex methodologies that would take significant work to interpret; (b) only a moderate priority for our research, since it has limited relevance to which charities we recommend and how we rank them.

Recently, we decided to explore the idea of working with David Roodman (whom we’ve generally been impressed with) as someone who might increase our capacity to produce thorough reviews of literature on key giving-relevant questions. For our first project together, we chose to work on this question because David had done substantial work on it in the past, and felt he could produce an initial writeup relatively quickly.