A chapter on animal models demonstrates the usefulness of such models to our understanding of temperament and mood. Several chapters are devoted to dopamine D4 and the serotonin transporter and their relationship to personality traits, novelty seeking, and temperament in early childhood. Dopamine pathways are thought to promote exploratory and impulsive behaviors, whereas serotonergic excitations generally foster avoidance. These findings are tentative given that the gene effect is weak. A section is devoted to the social and ethical implications of research on genes and personality. It reviews misperceptions common in the lay community and cautions against premature application of genetic findings to social policy. The editors do not shy away from controversial issues; they include a chapter on the genetics of sexual orientation and one on general intelligence. A chapter on autism demonstrates the approach taken in this book: Autism, as defined by DSM-IV-TR, is a lifelong, pervasive behavioral disorder characterized by broad language deficits, poor social interactions, and ritualized, repetitive behaviors. It has an oligogenic (a few genes with marked effects) inheritance pattern, several susceptibility loci on chromosomes 7 and 15, and a concordance rate of 36 to 91 percent among monozygotic twins, in contrast with none among dizygotic twins. Some relatives of autistic children possess genetically transmitted personality and language defects similar to, but milder than, those of autism. Their syndrome, called broad autism phenotype (BAP), can serve as a model for genetic determinants of personality. People with BAP are a defined group with inherited personality traits. Unfortunately, the phenotype is complex; it is associated with a range of neuronal abnormalities rather than a single brain disorder, and its symptom categories overlap.