People who believe Princess Diana was murdered or that John F. Kennedy's assassination was an elaborate plot are more likely to think that vaccines are unsafe, despite scientific evidence to the contrary, according to research published by the American Psychological Association.

"Vaccinations are one of society's greatest achievements and one of the main reasons that people live about 30 years longer than a century ago," said lead researcher Matthew Hornsey, PhD, of the University of Queensland. "Therefore, it is fascinating to learn about why some people are so fearful of them."

The study is the first to test the relationship between conspiracy beliefs and antivaccination attitudes among a global sample, according to Hornsey. The research was published in the journal Health Psychology.

Hornsey and his co-authors surveyed 5,323 people from 24 countries on five continents using online questionnaires between March 31 and May 11, 2016, measuring antivaccination attitudes and belief in four conspiracy theories: that Princess Diana was murdered; that the American government knew about the 9/11 attacks in advance and chose to let them happen; that a shadowy group of elites exist to plot a new world order; and that John F. Kennedy was murdered as part of an elaborate plot.

Those with strong beliefs in conspiracies were most likely to hold antivaccination attitudes regardless of where they lived. For example, the more people believed that Princess Diana was murdered, the more negative attitudes they had about vaccinations. In contrast, level of education had a very small impact on antivaccination attitudes.

"People often develop attitudes through emotional and gut responses," Hornsey said. "Simply repeating evidence makes little difference to those who have antivaccination attitudes."

Large pharmaceutical companies, which profit from selling vaccines, are often targets for conspiracy theorists, said Hornsey. "For many conspiracy theorists, profits gained are a sign that the system is broken and the truth is being covered up by vested interests."

"Trying to reduce people's conspiracy beliefs is notoriously difficult," Hornsey said. "An alternative possibility is to acknowledge the possibility of conspiracies, but to highlight how there are vested interests on the other side too; vested interests that are motivated to obscure the benefits of vaccination and to exaggerate their dangers."

Anti-vaccination attitudes were also associated with intolerance of those who limit their freedom, disgust toward blood and needles and an individualistic worldview, according to the study.