According to animal studies, the choice of a sexual partner is highly influenced by sex-specific pheromone signals, which are processed by male and female mating centers located in the anterior hypothalamus. And in a previous PET study involving humans, Ivanka Savic and colleagues at the Karolinska Institute in Sweden found that smelling AND (a testosterone compound) and EST (an estrogen compound) activated regions covering sexually dimorphic nuclei of the anterior hypothalamus, and that this activation was differentiated with respect to sex and compound. In a subsequent study, the researchers investigated the question whether the pattern of activation induced by AND and EST could be related to sexual orientation rather than to the biological sex. They compared the pattern of brain activation to a testosterone compound (AND) and an estrogen compound (EST)—candidate compounds for human pheromones—as well as other odors such as lavender and cedar in homosexual men and heterosexual men and women using positron emission tomography (PET) and functional magnetic resonance imaging (fMRI). Part 2 tomorrow.