Does uncertainty erode trust?

Water managers welcome uncertainty. But they hate communicating it to ordinary people. At least that’s what I found when I interviewed a dozen water supply planners back in 2014.

Suppose you work for a water authority. You’re in charge of telling bill payers about a proposed project with a price tag that looks like a small country’s coronavirus stimulus package. What’s your preferred communication strategy?…

Example 1 (no uncertainty communicated): “Our water supply system needs this new reservoir and transfers scheme because our current supply capability is 100 GL/yr and in ten years the demand will be 150 GL/yr, leaving a large deficit.”

Example 2 (uncertainty communicated): “Our water supply system needs this new reservoir and transfer scheme because without it there’s 5% probability that the 100-year drought would cause our reservoirs to empty, leading to a need for emergency measures that may or may not allow us to keeping supplying households with water.”

My intuition—and that of the practitioners I interviewed—says that bill payers are more likely to trust the planners’ investment decision if the need for it is communicated in straightforward terms, without uncertainty. But maybe our intuitions are wrong.

In the “posttruth” era where facts are increasingly contested, a common assumption is that communicating uncertainty will reduce public trust. However, a lack of systematic research makes it difficult to evaluate such claims.

This is from a new study by Anne Marthe van der Bles and colleagues at University of Cambridge. The work involves five separate experiments and was published yesterday in PNAS.

Results show that whereas people do perceive greater uncertainty when it is communicated, we observed only a small decrease in trust in numbers and trustworthiness of the source, and mostly for verbal uncertainty communication. These results could help reassure all communicators of facts and science that they can be more open and transparent about the limits of human knowledge.

The study focuses on science communication, but the conclusions are generalizable to any public communication situation.

I can say one thing for certain though: water utilities won’t be assimilating these insights into their public reporting any time soon.