Joseph Stalin said that "the death of one man is a tragedy. The death of millions is a statistic." There are many examples of this - the drowning of three-year-old Alan Kurdi grabbed the attention of the media in a way that the deaths of thousands of Syrian refugees before him had failed to do. This phenomenon - the identifiable victim effect - is well known in the charitable sector and many campaigns focus on single individuals.
In part, it's a simple problem that humans have with large numbers. Our cognitive systems are tied to our perceptions. Our brains are good at representing small numbers - which we perceive all the time - but less good at imagining large ones.
It has also been taken to mean that people give with their hearts not their heads. This puts emotion at the centre of motivations to give - but altruistic motives matter too.
Disaster appeals are a clear example where donors respond not to an identifiable victim but to an urgent, large-scale need. Kimberley Scharf of Warwick University, Mark Ottoni-Wilhelm of Indiana University and I have analysed the effect of Disasters Emergency Committee appeals using anonymised Charities Aid Foundation customer data, and we see that these appeals cause an immediate uplift in donations to charities involved in overseas relief.
The scale of the response to the appeals also maps onto the scale of the disaster - the more people who are affected, the bigger the response. Finally, donations to other charities are actually higher in the immediate aftermath of a disaster appeal, pointing to a wider positive impact.
One way to reconcile these two findings - the identifiable victim effect and the response to disaster appeals - is that donors want their donations to make a difference and both are ways of appealing to donors that they really can help. By contrast, statistics on mass suffering - particularly where problems are persistent and hard to solve - might leave donors thinking that their donations will have little effect. Mother Theresa (rather than Stalin) might have got closer when she said: "If I look at the mass, I will never act. If I look at one, I will".
Sarah Smith is professor of economics at the University of Bristol