We all need friends. Deeper and more numerous friendships promote health, well-being, survival, and even financial success. Impairments in the ability or motivation to connect with others profoundly impact the lives of individuals with neurodevelopmental and neuropsychiatric disorders. Despite its importance, the scientific study of friendship is relatively new, due to the difficulty of studying complex social behavior in the laboratory. Using a suite of brain imaging, eye-tracking, single-unit recording, pharmacology, and molecular neurogenomics to directly compare biology and behavior in humans and monkeys, his work has identified specialized circuitry that focuses attention on others, identifies their internal states, predicts their intentions, and promotes giving and cooperation.