1. Extreme Outcomes The Strategic Treatment of Low Probability Events in Scientific Assessment Anthony Patt Global Environmental Assessment Project Harvard University Scientific assessments that exist in order to enhance consensus among policy makers—such as the IPCC—show a clear pattern of not discussing low probability / high consequence events. This is consistent with a strategic model of assessment processes. Other assessments, such as narrow advisory ones, do not show this bias toward omitting extreme outcomes.

2. The study collected a random sample of 38 climate change assessments, published between 1982 and 1997. The study coded assessments as consensus seeking, advisory, or advocacy, based on intended audience, purpose, and process. The study examined a particular low probability risk: the rapid collapse of the West Antarctic Ice Sheet, causing catastrophic sea level rise. The study used multiple regression techniques, controlling for the assessments’ length, and year of publication. The study found significant results: consensus seeking assessments were likely to omit discussion of the risk; advisory assessments likely to omit discussion of the risk or give it extensive treatment, depending on their scope of work; advocacy assessments likely to give limited attention to the risk. Empirical Findings

3. Assessment Typology Consensus seeking assessments are designed to bring about agreement among policy-makers about wise courses of action in response to a problem. They are usually composed of a panel of recognized experts who represent a range of viewpoints. Their intended audiences are policy-makers and, through the media, the lay public. Advisory assessments are designed to inform a specific group of governmental of industry decision-makers. They are usually composed of internal scientists, or hired consultants from outside. Their intended audience is usually restricted to the group of decision-makers who commissioned their work. Advocacy assessments are designed to influence policy-makers and public opinion, in accordance with a particular organization’s political mission. They are usually composed of internal scientists, or hired consultants from outside. Their intended audience is the public at large, the media, and policy-makers. The important first part of this study, connecting assessment strategy to form, was the observation that there are three primary types of assessments.

4. Why Focus On Extreme Outcomes? People often make poor decisions in response to low probability possibilities. They either dismiss the risks as impossible, or else they overweight the risks in their own minds. Which way people respond to low probability risks depends in large part on the context in which they learn of the risk, the degree to which they can visualize the risk, and other factors. How an assessment discusses a risk—beyond a simple presentation of “just the facts”—can greatly determine whether people dismiss the risk, or fear it. Thus, we can learn a lot about assessments and their strategies by focusing on their discussion of extreme outcomes. Extensive work in the field of behavioral decision theory has shown that low-probability / high-consequence risks are qualitatively unique, in terms of how people interpret and react to them.

5. Consensus-Seeking Strategy We expect assessments from institutions like the IPCC or the National Research Council to omit discussion of low probability / high consequence risks, for two reasons. First, they will find it hard to achieve internal agreement on how best to discuss the risk, since small changes in how they present the risk could greatly influence how the audience responds to the information. Second, they will know that any discussion of the risk will generate debate among their intended audience, exactly opposite to their mission of generating agreement.

6. Advisory Strategy We expect assessments from governmental organizations like the Department of Energy, or industry assessments designed for internal review only, to cover low probability risks in great detail, if if falls within their assigned scope of work. These assessments will need to devote considerable attention to the risk in order to educate their audience about the risk, presenting multiple viewpoints, so that their audience can make an informed choice less dependant on how the risk is framed.

7. Advocacy Strategy We expect assessments from organizations such as the Global Climate Coalition or Greenpeace to devote limited attention to low probability risks. They will present one viewpoint about the risk, either to convince their audience to dismiss the risk as impossible (and hence discredit the other side as fear-mongering) or to fear the risk greatly. They will not devote too much attention to these risks, because that will make it easier for their opposition to discredit them.

8. More Information The April issue of Risk Decision and Policy has published these findings. The citation is: Patt, Anthony (1999). Extreme outcomes: the strategic treatment of low probability events in scientific assessments. Risk Decision and Policy 4(1): 1– 15. The author is continuing to study how scientists understand and communicate low probability risks, and he welcomes any comments or suggestions. This study is part of the Global Environmental Assessment Project at Harvard University. To learn more, or to contact the author, visit the project’s homepage. “www.environment.harvard.edu/gea”