Chronic traumatic encephalopathy (CTE) is a neurodegenerative disease associated with a history of repetitive head impacts (RHI). CTE was described in boxers as early as the 1920s and by the 1950s it was widely accepted that hits to the head caused some boxers to become “punch drunk.” However, the recent discovery of CTE in American and Australian-rules football, soccer, rugby, ice hockey, and other sports has resulted in renewed debate on whether the relationship between RHI and CTE is causal. Identifying the strength of the evidential relationship between CTE and RHI has implications for public health and medico-legal issues. From a public health perspective, environmentally caused diseases can be mitigated or prevented. Medico-legally, millions of children are exposed to RHI through sports participation; this demographic is too young to legally consent to any potential long-term risks associated with this exposure. To better understand the strength of evidence underlying the possible causal relationship between RHI and CTE, we examined the medical literature through the Bradford Hill criteria for causation. The Bradford Hill criteria, first proposed in 1965 by Sir Austin Bradford Hill, provide a framework to determine if one can justifiably move from an observed association to a verdict of causation. The Bradford Hill criteria include nine viewpoints by which to evaluate human epidemiologic evidence to determine if causation can be deduced: strength, consistency, specificity, temporality, biological gradient, plausibility, coherence, experiment, and analogy. We explored the question of causation by evaluating studies on CTE as it relates to RHI exposure. Through this lens, we found convincing evidence of a causal relationship between RHI and CTE, as well as an absence of evidence-based alternative explanations. By organizing the CTE literature through this framework, we hope to advance the global conversation on CTE mitigation efforts.

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The recent literature providing insights from neuroscience and evolutionary biology into how individuals perceive risky choice situations represents a "second wave" of findings that recapitulates as well as challenges the risk perception research begun in the 1980s, which relied on psychometric survey research. Gleaning insights from the first wave of research that could improve the communication and control of environmental risks has yielded disappointing results. This is a result, in part, of the eagerness of scholars and pundits to posit a chasm between the "rational" and "objective" perceptions of experts, on the one hand, and a lay public that is seen as lurching between "paranoia and neglect" and as insensitive to the magnitude of risks. Interpretations of the psychometric research have suffered from inattention to uncertainty and interindividual variability in risk, to expert biases, and to important aspects of risky choice that were not explored in the first wave of research. Initial signs indicate that neuroscience and evolutionary biology research may fall prey to similar misinterpretations. This article summarizes some of the most intriguing findings of the "second wave" of risk perception research and advances four themes that may help make the new findings less divisive and more useful for improving risk communication and risk management. Continued research into risk perception should perhaps be embedded in a more general theory of public choice in the face of uncertain and variable costs and benefits and with a respect for distributive justice as an important goal in risk management.