Yet as soon as Greece was unified into an empire by a thug – Philip of Macedon in 338 BC – it lost its edge. - Matt Ridley

Stress can alter the expression of genes, which can affect the response to stress and so on. Human behavior is therefore unpredictable in the short term, but broadly predictable in the long term.

There are two ways to tell the story of the twentieth century. You can describe a series of wars, revolutions, crises, epidemics, financial calamities. Or you can point to the gentle but inexorable rise in the quality of life of almost everybody on the planet: the swelling of income, the conquest of disease, the disappearance of parasites, the retreat of want, the increasing persistence of peace, the lengthening of life, the advances in technology.

If reproduction has been the reward and goal of power and wealth, then it is little wonder that it has also been a frequent cause and rewarding of violence.