Twelve-year-old Jaysen Carr died in July 2025. While he swam in Lake Murray, a reservoir a few miles from Columbia, South Carolina, Naegleria fowleri – a rare amoeba found in warm fresh water – entered through his nose, causing a rapidly fatal brain infection.
Each year in the United States, drowning causes roughly 4,500 deaths, while infections from brain-eating amoebas typically number only two or three. Yet the vividness of these rare deaths powerfully shapes how people perceive and respond to risk. After a 2025 amoeba-related death made headlines in Iowa, for example, open-water swimmers began questioning whether lakes were safe, even as health officials emphasized how rare such infections remain.
Is it irrational to avoid swimming in lakes on hot summer days? How rational is it to fear flying? How many people worry about contaminants in their drinking water yet never think twice about skipping sunscreen, despite skin cancer being the most common, and largely preventable, cancer in the United States?
These reactions raise a deeper question: What does it mean to call a response “rational” or “irrational”? These are the kinds of ideas I explore in my research on behavioral public policy. How do the assumptions scientists make about human rationality shape the tools governments use to improve social welfare?
When mistakes aren’t really mistakes
Behavioral economists, following Daniel Kahneman, emphasize how heuristics – the mental shortcuts or rules of thumb people use to make quick decisions – produce systematic biases or predictable errors in judgment. From this perspective, these biases born from shortcuts lead people to make choices that do not serve their own interests or stated preferences.
Evolutionary psychologists such as Gerd Gigerenzer instead see those same shortcuts as adaptive responses to uncertainty. Rather than errors, they’re efficient strategies shaped by the environments in which human reasoning actually evolved.
These two perspectives are in disagreement about what counts as rational – and why that matters for policy.
How a care team frames the risks of a procedure affects a patient’s choice.
Halfpoint Images/Moment via Getty Images
Consider a few familiar examples. Frame the same medical procedure as having a 90% survival rate rather than a 10% mortality rate and patients respond very differently. Set one option as the default – whether in organ donation, retirement savings or privacy settings – and most people stick with it simply because opting out takes effort.
From a behavioral economics perspective, these are clear cases of bias: judgments shaped by framing, whatever feels most vivid, or inertia rather than careful deliberation.
From an evolutionary perspective, however, the picture changes. In complex environments with limited time, information and attention, relying on defaults or whatever feels most vivid or familiar can be an…
