Why are conflicts between different human tribes so prevalent and why is violent intergroup conflict almost exclusively the domain of men...both as perpetrators and victims?

The latest tragedy in Egypt in which two soccer hooligan groups came to blows, leaving more than 70 fans dead is a case in point.

Our latest research on the male warrior hypothesis, which was published last week in Philosophical Transactions of the Royal Society, the oldest science journal in the world, offers an explanation.

Based on a review of the literature we argue that men may be biologically programmed to be warriors We dubbed this the male warrior hypothesis. There has been a lot of interest in our research. And, as you would expect, there have been criticisms too. Here I will try to separate fact from fiction and science from ideology.

The article on the male warrior hypothesis, co-written with Melissa McDonald and Carlos Navarrete from Michigan State University, looks into how the human psychology concerning war and conflict has been shaped by our evolutionary past. Following a review of the academic literature from social psychology to anthropology and political science to evolutionary biology, we conclude that men maybe biologically programmed to be warriors because of a deep ancestral history of inter-tribal conflict, perpetrated by adult males.

Including the results of studies that we conducted ourselves, we find that across different cultures and times, men are, on average, more likely than women to demonstrate prejudice and discrimination particularly against men viewed as outsiders.

We also show that men much prefer group-based social hierarchies—the outcome of intergroup conflict—and more strongly identify with tribal groups than do women.

When we ask men and women to name their favourite colour and explain why no fewer than 30 percent of males vs. none of the females come up with a tribal reason for their colour preference (e.g., the colour red, because it is the colour of my favourite football team).

Further we show that men are more motivated to defend their group if they are competing against another group. Thus, it seems that wherever you look men are more tribal than women. The question is why.

We hypothesize that for men, despite the significant risks, there have been various evolutionary benefits associated with organized intergroup violence, such as a greater access to resources, status, and  perhaps sexual mates. In contrast, women might on the whole be better off avoiding outgroup males.

There have been various criticisms on our findings, and one of the staunchest attacks came from a fellow PT blogger, the moral philosopher Prinz.

He argues that male intergroup aggression is a cultural manifestation of recent historical force: Farming techniques enabled men to control resources and dominate women. Essentially Prinz denies the existence of innate psychological differences between men and women.

It is disappointing that despite the evidence from thousands of scientific studies into sex  differences in social behaviour (including recent neuroscience findings) there are still people adhering to what my fellow psychologist Steven Pinker has called the "blank slate" perspctive on human nature.

Exactly what's wrong with Prinz' arguments.

First, intergroup aggression predates agriculture by many tens of thousands of years. In hunter-gatherer societies men (not women) practice tribal warfare, like the Papuas in New Guinea, leaving some 10-30 percent of men dead. The archaeological evidence is also quite clear, mass graves containing the skeletons of men (and just a few women and children) who died a violent death from arrows and spears have been dated back at least 30,000 years ago.

Male coalitional violence is also practiced by the chimpanzee, our closest genetic relative (we do not know enough yet about the bonobo, but the males are more suspicious to outsiders than the females).

Second, sex differences are real and they are as much about psychology as biology (as if these can somehow be separated!). Most behavioral scientists realize by now that "evolution does not stop at neck" and that natural selection can produce both differences in physique—think of the very stable sex difference in height—and in underlying psychologies. Any parent lucky enough to have both a son and a daughter will know what I mean. That there are innate differences between men and women in some psychological traits, including physical aggression, is so obvious that it is completely unscientific to state otherwise.

The third problem with Prinz' "cultural" view on male intergroup aggression is that it creates a false dichotomy between biological and cultural explanations, as if history and biology are somehow alternatives. Evolutionary approaches assume that history plays a significant role in shaping these male warrior tendencies. And, culture is a product of biology too because our capacity for cultural learning is surely innate.

Cultural factors can either exacerbate such male warrior tendencies (such as in highly militaristic societies like ancient Sparta) or attenuate them (the Netherlands springs to mind), but they are still there because they are part of an evolved male psychology.

To deny the existence of this aspect of the male psychology is just silly. It suggests that there are cultures in which the guys stay at home and the girls fight each other to death in violent tribal conflicts.

If anyone can point me to the existence of such societies I will eat my hat and completely give up on the male warrior hypothesis! 

(Before you respond, please note that the notorious Amazonian warrior cults are mythical).

Be sure to read the following responses to this post by our bloggers:

Sex and Violence: Male Warriors Revisited is a reply by Jesse Prinz Ph.D.

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