The book features many diagrams of brains, pointing out various regions that are active during different cognitive tasks. In general the correlations of active areas to cognitive tasks can be very useful to better understand the brain structures, if not to actually understand how the cognitive tasks are achieved. Most illuminating are the findings where either the same area is used during different types of tasks, or where different areas are used for what seem to be very similar tasks. I think it's probably valuable to combine these types of findings with traditional psychology to see what may be illuminated.
Lieberman's key claim is that our 'default' brain mode is used for so-called 'mentalizing' - sorting through the social world, trying to understand other people's motives and intentions. This is shown by the activation of certain brain areas both while explicitly thinking about social problems and when not attempting to do other cognitive tasks.
We typically use a particular prefrontal brain region for general cognition (reading, memorizing, computing, etc.), and it was thought that these areas were the critical to all learning. But various studies have found a 'social encoding advantage' in learning using the mentalizing system to form overall impressions of people and their intentions rather than simple memorization of people's behavior. The finding was that 'the folks making sense of the information socially have done better on memory tests than the folks intentionally memorizing the material.' (284) From the neuroscience angle:
Jason Mitchell, a social neuroscientist at Harvard University, ran an fMRI version of the social encoding advantage study. As in a dozen studies before his, he found that when people were asked to memorize the information, activity in the lateral prefrontal cortex and the medial temporal lobe predicted successful remembering of that information later on. According to the standard explanation of the social encoding advantage, the same pattern should have been present or event enhanced when people did the social encoding task, but that isn't what happened. The traditional learning network wasn't sensitive to effective social encoding. Instead the central node of the mentalizing network, the dorsomedial prefrontal cortex, was associated with successful learning during social encoding. (284-5)Lieberman suggests a number of interesting applications of this finding to change and hopefully improve the way we teach kids, who are intensely interested in the social world and not so interested in memorizing facts - such as by teaching history more in terms of the social dramas (rather than actions and dates), and math by engaging students as both tutors and tutees.
The book has sections on three stages of social development, which he terms connection, mindreading (theory of mind), and harmonizing - and argues that significant brain resources are devoted to maintaining connection with other people. Harmonizing is about taking on many of the goals and behaviors of our social group (particularly active during adolescence). The idea here is that our sense of self as supported in the brain is very susceptible to the social messages we receive.
Overall I liked this book - not that it really lives up to the subtitle 'Why Our Brains Are Wired to Connect' - it's more about 'How' than 'Why'. At its best it reminds us that we are truly social creatures, and the neuroscience helps illustrate that point.