Neurochemicals change the way our nervous system processes information from our environment and help us interacting optimally with it. These include neurotransmitters like dopamine, which plays an important role in maintaining normal levels of motivation, but also sex hormones like testosterone, which is responsible for maintaining e.g. a normal sex drive. In order to study the neurobiological basis of certain behaviours such as complex psychological phenomena like trust, it is insightful to study the effects of an experimental manipulation of those neurochemicals. Because most of them cannot cross the so-called “blood-brain barrier”, direct oral or intravenous administration is generally not possible. Neurotransmitter and hormone systems can however be influenced by drugs that are tailored to cross the blood-brain barrier and act on the receptors of these systems. In order to draw specific conclusions about the role of a given neurochemical in modulating a behavior, we usually compare a group of individuals that have had their neurochemical levels modified by a drug with a control group, that received a placebo, which has no pharmacological effect.