The Rules of Contagion: Why Things Spread - and Why They Stop

by Adam Kucharski

If R is 10 in a fully susceptible population, we’d need to vaccinate at least 9 in every 10 people. If R is 20, as it can be for measles, we need to vaccinate 19 out of every 20, or over 95 per cent of the population, to stop outbreaks. This percentage is commonly known as the ‘herd immunity threshold’. The idea follows from Kermack and McKendrick’s work: once this many people are immune, the infection won’t be able to spread effectively.

R = Duration × Opportunities × Transmission probability × Susceptibility

From a Bayesian point of view, we are generally better at judging the effect of arguments that we disagree with.[67]

So why hasn’t the CDC done more research into gun violence? The main reason is the 1996 Dickey Amendment, which stipulates that ‘none of the funds made available for injury prevention and control at the CDC may be used to advocate or promote gun control.’

easy access to deadly methods can make a difference for what are often impulse decisions. In 1998, the UK switched from selling paracetamol in bottles to blister packs containing up to thirty-two tablets. The extra effort involved with blister packs seemed to deter people; in the decade after the packs were introduced, there was about a 40 per cent reduction in deaths from paracetamol overdoses.[49]

The problem is that the most effective solutions may not be those that people are most comfortable with. Do we want to feel like we’re punishing bad people, or do we want less crime? ‘When it comes to behavior change, threats and punishment are just not that effective,’

‘What is ultimately most effective at changing a person’s behavior is when you try to sit down and try to listen to them and hear them out, let them air their grievances and really try to understand them,’