In his ambitious and compelling book, David Traven makes the case that international humanitarian laws of war are grounded in innate, universal moral sentiments backed by emotional impulses. In contrast to realists, who view humanitarian restraints on the conduct of war as reflecting national interests, and constructivists, who tend to view norms as socially constructed all the way down, Traven argues for a more “naturalistic” account of norm emergence rooted in intrinsic moral psychology. Drawing on recent research in psychology and neuroscience, the book describes how empathy and perspective-taking are universal moral intuitions, evidenced in children too young to be shaped by culture or societal expectations. These traits, in Traven’s viewing, are essential to understanding the emergence and endurance of similar humanitarian laws of war across time in a diverse array of cultures because, without them, humans would have little capacity for social coordination, altruism, or the desire to protect others from harm.

Traven adds further complexity to his argument, claiming that our evolved moral psychology explains not only the widespread emergence of humanitarian laws of war aimed at protecting civilians against intentional attack but also why we see violence against civilians when we do. Universal moral distinctions that perceive more harm in intentional killings than unintended killings serve to permit higher levels of civilian causalities as a byproduct of war than there might be if humans had evolved with a different set of moral templates.

On the whole, I learned a tremendous amount from this book. I found much of Traven’s argument about universal moral sentiment serving as a basis for the emergence of similar views of conduct during war to be highly compelling. To be frank, I was a pretty easy target for the argument to begin with and largely agreed with the claims before ever reaching the empirical chapters. The experimental evidence for the universality of certain moral sentiments, including empathy and intentionality, among children appears robust, convincing, and profound. (In their defense, constructivist have at times acknowledged that universal morals might serve as the basis for resilient norms, as the book indeed acknowledges on p. 29).

Although the historical chapters of the book did not necessarily sway my views, it is interesting to consider whether they would convince a skeptic. The book spends considerable time examining the emergence of thought about the proper regulation of conduct during war in three historical eras: the Warring States period in China (453–221 BC), the seventh- and eighth- century emergence of Islamic law, and the emergence of natural law theorists in tenth- and eleventh-century Europe. The book examines these cases not for concrete demonstration but rather for demonstration of the plausibility of the claim that a shared, affect-backed moral grammar exists across time and place and that these moral intuitions can shape reasoning about the conduct of war. In each of these eras, Traven finds evidence that theorists or statesmen of the day made affect-backed moral arguments in defense of the protection of civilians, even as they, in some cases, also rooted their arguments in the more instrumental terms that realists might expect.

I suspect that skeptics will be largely unconvinced of the book’s claims by what could appear as cherry-picked quotes from three historical eras. How do we know that such statements are not evidence of some random draw in which we would expect all sorts of sentiments and statements to find places in the historical record? Might these cases not share some yet undefined cultural similarities that actually drive the occasional expression of moral-seeming sentiment? I think the book could have done a bit more to defend itself from some of these challenges by discussing how we should think about the evidentiary basis for a universal trait that affects human behavior at the highest organizational level. I believe the reader would also have benefited from more systematic description of who were the particular theorists, how representative they were of a given body of thought, and what roles they played within the political and intellectual structure of the day. Without this, the claims that their thoughts influenced practice and behavior often felt tenuous.

Since I am among those convinced by the fundamental argument that shared moral sentiment can give rise to similar attitudes and practices about the conduct of law, however, I will discuss where the acceptance of this claim leaves us. Traven is clear that moral sentiment does not determine behavior. Rather, the mind “comes equipped with cognitive and emotional bases that lead people to endorse particular norms,” but these bases do not explain norm emergence itself. The book also claims that “material and cultural forces” (p. 73) are indeterminate in explaining which norms arise when and where. It shows, for example, that particular political and economic forces are not necessary for the emergence of empathic norms. The process of state formation seems to have been central to the story of humanitarian considerations in the case of Warring State China. In their need to expand the size of the military, leaders were forced to rely more on peasants who would never have previously figured into their notion of a protected in-group, and the expanded exposure to and reliance on a wider class of people paved the way for an expanded sense of in-group. But changes in state structure seem to have played little role in the rise of humanitarian norms in Islamic law or in latter-day Europe, suggesting that such structural changes are not a necessary condition for changes in moral sentiment that generate shifts in thinking about the conduct of war.

This leaves the reader wondering how we are to understand our observed world differently after reading this book. Humans share a range of universal moral sentiments, including disgust and resentment, as the book notes. Demonstrating the universality of a moral intuition like empathy is a first step. But how far does this argument alone get us if we do not then understand what causes the expression of one moral intuition in a time or place? Empathy is malleable. It can be applied to the nearby few or to humanity as a whole, as the book notes. “Culture and tradition” play a key role in determining the target of one’s empathy. Leaders may persuade us to adopt a more expansive notion of our in-group, and thereby the circle of people we believe are deserving of empathy and protection. But these forces are always seemingly in competition with moral components like resentment and disgust that push us to tighten and defend our circle from wrongdoers. What then determines which moral sentiments are activated for long enough to shape policy in the international domain? Can we not say anything more specific about when we should expect empathy and perspective-taking to shape international laws and practice, rather than more corrosive forces that are just as universal?

In addition, if charismatic politicians and persuasive rhetoric appealing to our moral and emotional intuitions play such a key role in determining the target of our empathy and the number of people we perceive as deserving of protection from harm, how far have we advanced past the constructivist argument? The book suggests that a key contribution of incorporating a more naturalistic element into norm creation is the ability to explain why certain normative content endures. But I am not sure that this more naturalistic framework necessarily explains this. If leaders have a full range of universal moral sentiments to appeal to through emotive rhetoric, why would we expect any one of those moral sentiments to have enduring resonance and effect on practice and behavior? Could not domestic political incentive structures then be playing a primary motive for norm emergence after all?

Ultimately, the book need not answer these questions to make several significant contributions. In its approach, it provides a clear model for how to effectively draw on advances in neuroscience and moral psychology to deepen our understanding of international attitudes toward violence. By demonstrating the existence of norms protecting civilians across cultures as diverse as ancient China and medieval Islam and by showing that the arguments for these norms would resonate with those made by European natural law theorists centuries later, the book upends the common view that international humanitarian law is a unique byproduct of Western European thought. Much work remains to understand the profound implications of this argument. I hope this book inspires many others to take up the challenge.