Social media chat on anti-vaccination Facebook pages is led by a tone of moral outrage, plus a strong belief in the oppressive nature of governments and the media. What's more, the vast majority of participants are women.
That's according to a new study of six public anti-vaccination Facebook pages, with hundreds of thousands of Likes between them, using a variety of data analysis methods.
While these pages cover a wide area in terms of geographical regions, the people doing most of the commenting actually represent a much smaller subset, the research reveals, suggesting social media plays a key role in fanning the flames of anti-vaxxer feeling – and in getting people to ignore the weight of evidence.
"Understanding pockets of resistance to vaccination as a public health exercise provides important insights into how these attitudes may be effectively countered," write the researchers in their paper.
"Effective disease prevention is contingent on high levels of vaccination compliance and coverage within networked populations."
Previous research has shown that anti-vaccination websites make much better use of interactive tools like comments, forums, and social media, than pro-vaccination sites, which tend to act more like static libraries of information.
In other words, when you're searching for health information on the web, the people shouting loudest can often drown out the people talking sense.
To dig deeper into the online anti-vaxxer communities, Naomi Smith from Federation University Australia and Tim Graham from Australian National University looked at the structure of the six hand-picked Facebook pages, the gender balance, and the main recurring discussion topics.
The pages analysed were Fans of the AVN, Dr. Tenpenny on Vaccines, Great Mothers (and Others) Questioning Vaccines, No Vaccines Australia, Age of Autism, and Rage Against the Vaccines.
In total, these pages had 231,491 Likes at the time the study was carried out (December 2015).
By scraping three years' worth of data, the researchers found that most of the page followers had only liked posts once or twice – the vast majority of the online activity was generated by a smaller group of people. Commenting activity also suggested most users were transient or just occasionally passing through the pages.
Meanwhile, using a maths modelling approach known as the Latent Dirichlet allocation, the researchers also picked out the most common topics on these boards: activism, governance, media and censorship, and vaccination as genocide all scored highly.
Other topics frequently mentioned were the Zika virus, the (pro-vaccine) Gates Foundation, moral transgressions, vaccine injuries, and chemtrails (the basis of another popular conspiracy theory).
While the study was more of a fact-finding mission than one designed to present any firm conclusions, Smith and Graham do point out that these Facebook pages aren't particularly close-knit but can be useful in reinforcing beliefs through a filter bubble effect.
Meanwhile, the gender ratio suggests it's mothers rather than fathers who are tackling the question of vaccinations and looking up information about it online.
Finally, the list of topics found by the researchers shows strong anti-government and anti-media sentiments in these pages. In other words, conspiracy-style beliefs abound.
If we're going to understand more about the anti-vaxxer movement and how to change some minds, more research of this type is needed, say Smith and Graham.
"Concerns about vaccination reveal a community that feels persecuted and is suspicious of mainstream medical practice and government-sanctioned methods to prevent disease," conclude the researchers.
"In a generation that has rarely seen these diseases first hand, the risk of adverse reaction seems more immediate and pressing than disease prevention."
The research has been published in Information, Communication & Society.