Classic debates in International Relations have touched on how humans tick. For instance, competing views of human nature have clashed in the 'Greed versus Grievance' debate of the early 2000s, as well as the timeless haggling between rationalist and constructivist explanations for war. Insights from neuro-psychological research begin to inform such debates, for instance in Steven Pinker's Better Angels, Yuval Noah Harari's Sapiens, and Robert Sapolsky's Behave. Barrett's book does not try to do this explicitly, but it is a valuable collection of insights for students of conflict. The book starts off by explaining the evolutionary origins of central nervous systems: allocating resources for foraging and predation. Barrett intuitively conveys under which ecological conditions such capabilities started to pay dividends hundreds of millions of years ago. Chapter four introduces the 'predictive brain' hypothesis and explains how it relates to studies on prejudice and discrimination, as well as unintentional attacks on civilians in war. It also discusses the possibility of building cross-cultural connections through shared experiences, in line with the long-standing 'contact hypothesis' in social psychology. The power of words to inflict stress on the human body is discussed in Chapter five, which builds another bridge between contentious politics and health outcomes. In summary, this book is a short and very accessible treasure trove for anyone interested in the neural underpinnings of social phenomena.