It is called The Moral Molecule: The Source of Love and Prosperity. Zak coined the term neuroeconomics but this book seems to be about so much more than the title suggests. I doubt I can convey how interesting and well written this book is (I may not be totally objective since I am among the people thanked for their help in the acknowledgements).
Zak wrote a good summary article in The Wall Street Journal. See The Trust Molecule.
The "moral molecule" is oxytocin and the book explains many experiments that show that we tend to be more trusting when it is present or increases (how oxytocin affects us and works with other hormones and neurotransmitters is alot more complex than this, though, as Zak's work shows). How this is related to empathy is discussed and this is where Adam Smith comes in (his theories on sympathy).
The book examines when and why people are nice and when they are not. When are we altruistic and when are we selfish? The role that evolution played is examined. How this all affects us socially and politically is discussed. Zak is an expert on how trust is a key ingredient to the success of economies and trust is related to oxytocin. His experiments show what happens when people have their amount of oxytocin increased. How this is all related to relgion is discussed.
The book is also full of humorous anecdotes and personal stories. It is highly entertaining and thought provoking. The insights into human nature are amazing. I especially liked the discussion of "in-groups" and "out-groups." You don't have to be a neuroscientist or economist to understand it and it might be a great book for professors to assign to undergraduates.
It received an excellent review in The Wall Street Journal. See Kin and Kindness by MICHAEL SHERMER, publisher of Skeptic magazine and a monthly columnist for Scientific American.
Here are two articles about professor Zak's 2010 Mind Science Foundation lecture from the San Antonio Express-News:
Emerging field offers insight into human virtues
Humans release ‘niceness' chemical
More information about neuroeconomics can be found at (these are two short articles by Zak that give you some idea of what he does in his experiments):
Neuroeconomics Explained, Part One
Neuroeconomics Explained, Part Two
Adam Smith vs. Bart Simpson. (A post of mine from last year and it also has a link to a video of Zak lecturing on all of this)