In neurology and neuropsychology, behavior refers to the way human beings act and make decisions in contact with their environment. Behavioral impairment is therefore defined as a pathology, following brain lesion, that impacts the interactions between the brain-lesioned individual and his/her surrounding social world. First descriptions of behavioral disorders, including neuroanatomical correlates, date back to the mid-19th century. However, attempts towards their systematic identification and analysis only began at the turn of the 19th to 20th century. In this chapter, we shall span 3 main themes by introducing the first case reports based on thorough clinical descriptions, dating back to the 19th century. We then examine the emergence of checklist questionnaires and their application to large cohorts of individuals starting after World War II. Finally, we outline how, over the last 3 decades, the pace has significantly accelerated in the pursuit of defining the fine-grained processes underlying behavioral functioning, as well as the development of new and more complex measures, along with the emergence of the social cognition and social brain concepts. As the assessment tools have expanded and become more specific, an increasing complexity of mechanisms underlying behavior has begun to emerge.
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