One answer is that contemporary America suffers from a dangerous lack of historical memory. Most of the parents who are today skipping or delaying their children’s combined measles, mumps, and rubella (MMR) vaccine don’t remember life with measles, much less that it used to kill more children than drowning does today. Nor do they recall how other diseases stamped out by vaccines—most prominently smallpox and polio—took lives and disfigured bodies.

Read: Measles and the limits of facts

Our amnesia about vaccines is part of a broader forgetting. Prior generations of Americans understood the danger of zero-sum economic nationalism, for instance, because its results remained visible in their lifetimes. When Al Gore debated Ross Perot about NAFTA in 1993, he reminded the Texan businessman of the 1930 Smoot-Hawley Tariff Act, which raised tariffs on 20,000 foreign products—prompting other countries to retaliate, deepening the Great Depression, and helping to elect Adolf Hitler. But fewer and fewer people remember the last global trade war. Similarly, as memories of Nazism fade across Europe and the United States, anti-Semitism is rising. Technology may improve; science may advance. But the fading of lessons that once seemed obvious should give pause to those who believe history naturally bends toward progress.

Read: The anti-vaccine movement is forgetting the Polio epidemic

Declining vaccination rates not only reflect a great forgetting; they also reveal a population that suffers from overconfidence in its own amateur knowledge. In her book Calling the Shots: Why Parents Reject Vaccines, the University of Colorado at Denver’s Jennifer Reich notes that starting in the 1970s, alternative-health movements “repositioned expertise as residing within the individual.” This ethos has grown dramatically in the internet age, so much so that “in arenas as diverse as medicine, mental health, law, education, business, and food, self-help or do-it-yourself movements encourage individuals to reject expert advice or follow it selectively.” Autodidacticism can be valuable. But it’s one thing to Google a food to see whether it’s healthy. It’s quite another to dismiss decades of studies on the benefits of vaccines because you’ve watched a couple of YouTube videos. In an interview, Reich told me that some anti-vaccine activists describe themselves as “researchers,” thus equating their scouring of the internet on behalf of their families with the work of scientists who publish in peer-reviewed journals.

In many ways, the post-1960s emphasis on autonomy and personal choice has been liberating. But it can threaten public health. Considered solely in terms of the benefits to one’s own child, the case for vaccinating against measles may not be obvious. Yes, the vaccine poses little risk to healthy children, but measles isn’t necessarily that dangerous to them either. The problem is that for others in society—such as children with a compromised immune system—measles may be deadly. By vaccinating their own children, and thus ensuring that they don’t spread the disease, parents contribute to the “herd immunity” that protects the vulnerable. But this requires thinking more about the collective and less about one’s own child. And this mentality is growing rarer in an era of what Reich calls “individualist parenting,” in which well-off parents spend “immense time and energy strategizing how to keep their children healthy while often ignoring the larger, harder-to-solve questions around them.”