Exposure refers to the fact that people tend to underestimate the cumulative effect of multiple exposures to a risk (Linville et al., 1983). In many instances of risk, the concern is about exposure over time, not necessarily from a single exposure alone. Communication of cumulative risk can be helpful in these instances. Cigarette smoking is an example of an exposure in which cumulative risk is important.

Comparisons. Risk is multidimensional, but when a communicator makes a risk comparison on the basis of one or two dimensions, people may assume that many dimensions are being compared and draw conclusions based on the broader comparison rather than that which was intended. For instance, experts may say that the risk of an environmental exposure is inconsequential because on average it is low, but ordinary people might call for action because they fear that the risk falls disproportionally, and thus unfairly, on vulnerable groups.

Omission bias is the tendency to believe that an error of omission is less serious than an error of commission. That is, people tend to be more averse to a risk incurred by taking an action than one incurred by taking no action. For example, a University of Pennsylvania study found that nonvaccinators (parents who chose not to vaccinate their children) were more likely to accept deaths caused by a disease (that is, omitting vaccination) than deaths caused by vaccination (an act of commission) (Meszaros et al., 1996).

Framing, the way in which information is presented or the context into which it is placed, affects how risk communication messages are received. Studies show that a different framing of the same options can induce people to change their preferences among options (Tversky and Kahneman, 1973; Lichtenstein and Slovic, 1971). This is known as a preference reversal. For example, the data on lung cancer treatment suggest that surgical treatment has a higher initial mortality rate but radiation has a higher 5 year mortality rate. In one illustration, 10 percent of surgery patients die during treatment, 32 percent will have died one year after surgery, and 66 will have died by five years. For radiation, 23 percent die by one year and 78 die by five years. When people are given these mortality statistics, they tend to be evenly split between preferring radiation and preferring surgery. When the same statistics are given as life expectancies (6.1 years for surgery and 4.7 years for radiation) there is an overwhelming preference for surgery (McNeil et al., 1982).

How information is framed can also affect whether people allow an omission bias to be a prime motivator of a decision not to vaccinate. One study of university students found that when the issue of responsibility was removed, subjects were more likely to opt for vaccination. Responsibility was removed by reframing the question as "if you were the child, what decision would you like to see made" (Baron, 1992).



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