There has recently been quite a bit of scientific research on the evolutionary benefits of kindness. This article (found via onegoodmove.org) summarizes some of the work quite nicely. Again the theme is reciprocity; when we are kind to others, others tend to do more for us:
“The findings suggest that anyone who acts only in his or her narrow self-interest will be shunned, disrespected, even hated,” Willer said. “But those who behave generously with others are held in high esteem by their peers and thus rise in status.” – Anwar (2010)
The article links to UC Berkley’s Greater Good Science Center and in particular, their Science for Raising Happy Kids website. Despite the somewhat Orewellian name (thanks Simon Pegg), the website has a lot of good information and a very good blog. I particularly liked a recent post by Christine Carter on Five Ways to Raise Kind Children. What she proposes aligns very much the Montessori philosophy.