We examine behavioral and physiological aspects of primate emotional attachments in the context of four relationships: infant-to-parent, parent-to-infant, and adult male-to-female and adult female-to-male in a monogamous New World species. Emotional attachments in each of these relationships show striking similarities at a basic functional level. The nature of these similarities suggests that they are produced by the same psychoneuroendocrine core, which appears to be present in all mammals. We also consider the development of each of kind of attachment. In contrast to fundamental similarities in the expression of attachment, their development in each case appears to be based on distinct, species-typical dispositions and constraints.