Proxemics
The study of personal space:

Don’t Stand So Close To Me. In 1966 anthropologist Edward T. Hall coined the term proxemics to describe the study of how people perceive the proximity of others. Hall’s work was inspired by an animal study conducted by German zoologist Heini Hediger, who found that animals maintained various boundaries depending on whether they were preparing to escape, to attack, to communicate with members of another species, or relating to a member of their own species.
Based on these insights, and after conducting his own research, Edward Hall developed the idea of a set of expanding circles, called reaction bubbles, that described how humans manage the space around them. The innermost circle he identified as Intimate space, reserved for those we are closest to, and usually measuring 6 to 18 inches (15 to 45cm) in radius. The next level up he dubbed Personal space, the distance we are comfortable maintaining with close friends, about 1.5 to 4 feet (0.5 to 1.2m). He used the term Social space to indicate our preferred proximity to acquaintances, about 5–12 feet (1.5–3.6m), and Public space for the distance we need for public speaking, 12–25 feet or more (3.6–7.6m).
This sounds very specific, but Hall himself acknowledged that these distances vary from culture to culture. While those from less-populated countries, or countries where individualism and privacy are highly valued, are more comfortable with larger spaces between themselves and others, in other cultures maintaining what is considered excessive distance can be perceived as rude or unfriendly. I'm hooked on Wikipedia---;>)