This book is about the hormone oxytocin (which is principally a female hormone but also present in the male). This hormone is the molecule referred to in the title of the book. "Am I actually saying that a single molecule...accounts for why some people give freely of themselves and others are coldhearted bastards, why some people cheat and steal and others you can trust with your life, why some husbands are more faithful than others, and, by the way, why some women tend to be more generous - and nicer - than men?" asks the author Paul Zak. "In a word, yes" he answers.
This molecule as Zak calls it, is a "feel good" hormone that increases when we do simple, feel good things like giving or receiving a hug, or when we give generously. The act of giving stimulates this hormone resulting in the recipient desiring to trust the giver. Zak explains that there is also a counter hormone ("testosterone") which he calls the "bad boy" hormone that increases the impulse to take risk and behave badly. However, testosterone is necessary for physical courage and strength. Thus, the mammalian animal evolved with these two hormones balancing each other and so many of the unusual behavior Zak says, can be attributed to an imbalance of those hormones.
From the general effects and the origin of this hormone in the evolutionary process, Zak discusses specific topics such as the effect of oxytocin deprivation in orphans. He also discusses the influence of oxytocin inclining people to religion. Zak believes, however, that religion serves a useful purpose regardless of whether God exists or not. Some parts of his theory concerning the connections between oxytocin in early civilisations may be controversial insofar as he postulates that oxytocin influence on the feel good factor leads people to have warm feelings which are interpreted as "love" and its connection to sex, and from there to procreation and creation. So maybe for the religious God is love but for scientists like Zak, love is God. It is a fascinating and illuminating book on the whole if one is interested in what makes us tick.