Human life, scientific journalist Matt Ridley suggests, is a complex balancing act: we behave with self-interest foremost in mind, but also in ways that do not harm, and sometimes even benefit, others. This behavior, in a strange way, makes us good. It also makes us unique in the animal world, where self-interest is far more pronounced. "The essential virtuousness of human beings is proved not by parallels in the animal kingdom, but by the very lack of convincing animal parallels," Ridley writes. How we got to be so virtuous over millions of years of evolution is the theme of this entertaining book of popular science, which will be of interest to any student of human nature.