Key extract: The researchers classified human relationships into four types: in-roles (who include colleagues, classmates, and others who we are acqua

Key extract: The researchers classified human relationships into four types: in-roles (who include colleagues, classmates, and others who we are acquainted with and see on a regular basis), friends (those who we are acquainted with, but do not see regularly), strangers (those who we are neither acquainted with nor see regularly), and familiar strangers (those who we see regularly but are not acquainted with). The new study finds that familiar strangers may play a significant role in processes such as information sharing and disease spreading.