Cornelia Betsch and Katharina Sachse recruited 115 participants online (mean age 34; 34 per cent were male; 43 per cent had one or more children). The participants were asked to imagine they were a parent of an 8-month-old and to read an account of a fictitious illness Phyxolitis pulmonis. They were further told that their paediatrician had advised vaccinating their child against this condition. Next, the participants were presented with anti-vaccine statements that they'd ostensibly found on the internet (e.g. "Multiple vaccines overwhelm the infant's immune system"). Finally, they read statements of reassurance about the vaccine, which claimed any risks were low - half the participants read weak versions (e.g. "There is only sporadic evidence that repeated vaccinations overwhelm the immune system") and half read strong versions of these statements (e.g. "there is no evidence that repeated vaccinations overwhelm the immune system").
The key finding here was that participants who read the strong statements of reassurance actually reported greater perceptions of risk afterwards, and lower intentions to vaccinate their child. This effect was heightened among participants who had a preference for complementary medicine. Results didn't vary according to whether participants were a parent in real life or not.
A second study with a further 119 participants was similar but this time the source of the reassuring statements was varied, either being from a pharmaceutical company (untrusted) or from a government health department (a trusted source). Again, strong statements of reassurance backfired, increasing risk perception and reducing vaccination intentions, but only if those statements came from an untrusted source. Again, this paradoxical effect was stronger among participants who favoured complementary medicine.
This study can't reveal why the paradoxical effect occurs. However, one possibility proposed by Betsch and Sachse is that an extreme statement of no risk is more attention-grabbing, which only serves to highlight the possibility that risk is an issue. Another potential explanation is that people look for ways to combat claims they disagree with, and if those claims are stated more strongly then that encourages people to marshal even stronger counter-claims of their own.
The results have obvious implications for real-life risk communication. "Especially when organisations lack complete knowledge about how much trust the public puts in them, optimal risk negation is likely to profit from moderate rather than extreme formulations," the researchers said.
Betsch, C., and Sachse, K. (2012). Debunking Vaccination Myths: Strong Risk Negations Can Increase Perceived Vaccination Risks. Health Psychology DOI: 10.1037/a0027387
Previously on the Research Digest:
How to promote the MMR vaccine.
The psychological barriers facing MMR promotion campaigns.
Post written by Christian Jarrett for the BPS Research Digest.
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