The ambitious aim of this book, to elucidate Why we are who we are and do the things we do, rings a bit like a 1970s song lyric and, frankly, seems unattainable. But Sullivan, a professor of microbiology and immunology, gives it the old college try. He explains how human behavior, actions, and personality arise from a complex interplay between genes, epigenetics (the effect of environment on our genes), hormones, neurotransmitters, our evolutionary history, environment, our microbiota (the trillions of microorganisms living on and inside us), and culture. He examines how these many factors and forces contribute to moods and phobias, sex and love, taste and appetite, brain and mind, addictions and beliefs (religious and political). The scientific discussion is enlivened by Sullivan's quirky sense of humor and frequent nods to pop culture. Unusual (and sometimes unsettling) information, such as that about one-third of persons are infected by a single-cell parasite, Toxoplasma gondii, transmitted by cats or acquired from contaminated food or water, is plentiful. Humans' only innate fears are of falling and loud, unanticipated noises. Intriguingly informative.--Tony Miksanek Copyright 2010 Booklist