I dunno. But perhaps we should try and ignore our warrior-chimp ancestry and learn from the blessed, peaceful bonobos. At least that seems to be the idea behind Sex & War: How Biology Explains War and Terrorism and Offers a Path to a Safer World. Yes, I know what you're thinking: another trendy but implausibly sweeping sub-title. Nonetheless, I heartily* recommend this book and suggest it may make an ideal christmas present for more people than you might think.
Wired interview with the authors and book extract here.
*Have you read the book, Massie? No. So why the recommendation? Because one of the authors, Thomas Hayden, is a friend, silly.