An actually quite remarkable cultural phenomenon
which has lacked proper study, and hasn't even a proper word coined for it. It involves body language
and personal space
and can tell one quite a bit about how close one is to another by mere appearances. Not merely on a physical
level, but also on a psychological
level. The relationship between two individuals may be partially gleaned
and comprehended, by merely examining the distance between their two heads.
Since caveman times we human beings have developed a rather unconscious series of complex protective barriers. Our minds know implicitly how much space is required between ourselves and any other organism before we can safely act. This falls into the realm of fight or flight reflex action. Admit it. You will either purposefully or unconsciously keep a greater amount of space between yourself and a complete stranger than you would between a confidante or member of your family. Unless of course a member of your family has proven themselves to be worse than the average stranger, like say you happen to meet your sister at the doorway with a sinister look in her eyes and a very large blood-dripping cleaver hiding rather unsuccessfully in her hands behind her back. At such a moment you may find the space between your head and hers suddenly become vast indeed, as you run in the opposite direction.
In fact, many of the complex decisions the mind has aquired over the millenia have been proven to be false. Others have been proven to be justified, but it's arguable as to which of our deep-hidden social constraints are based on factual information and which are based on circumstantial evidence. For example, at one time it was deemed that those who were dextrous were individuals who were right-handed and being right-handed meant they were right for all intents and purposes while those who were instinctively left-handed were known as sinister and therefore of the Devil. We still have many instinctive tendencies even today that are echoes of that now dated theological assumption. Most still instinctively reach out with their right hand to greet others. Less than fifty years ago, school administrators and teachers were instructed to force children who were left-handed to instead use their right, because that was proper. Those who were deemed left-handed were sometimes forced to go against their own inclinations, in order to adhere to a socially ill-concieved concept of what was right and wrong. Similar socially ill-concieved concepts are still out there. Others may have more fact than fiction to them, and only through careful scrutiny and examination can we glean what is useful for survival, and what is merely hampering inevitable peace and harmony.
Provided you believe in such hogwash and inevitable peace and harmony. But I digress.
Most of the time, the space between two people's heads is much more subtle than running away from a sister with a bloody cleaver. Properly gauging and reading such distances would require complex measuring devices were we to perform proper scientific analysis. However, such tampering of situations out in the field would require adversely interrupting the flow of natural events, and so proper research of these theories is of course impossible. Kindly see the quantum theory the Heisenberg Uncertainty Principle for more information.
The best way to take note of this phenomenon is by simple observation. Two individuals you are familiar with who are each intimately familiar with one another will generally have less of a space between their heads than two individuals not intimately familiar with one another. The longer two individuals are comfortable with one another, the smaller the distance between their two heads, until theoretically eventually their heads will lightly connect. This is what we call kissing. Consequently, the distance between two who have already been closely intimate in private, may become adversely even farther away when in the company of individuals who are not aware of their heads having recently locked together, especially if there is a fear of some jealous boyfriend or girlfriend who might seek to knock someone's head off if they were to be found out, or in the case of a recent United States president; impeachment. Whatever the case may be.
In summary, two strangers will generally keep their heads at a distance from one another slightly more than those who have known each other beyond the point of strangers, and the longer two heads have known one another, the closer that distance between two heads will become. This distance has a tendency of slowly getting shorter and shorter over time, unless some physical or mental obstacle is introduced which impedes the inevitable flow of inertia. In fact it can be theorized that it is the natural inclination of humankind and the universe for all heads to eventually carefully lock in to one another, but that a very complex network of physical and psychological obstacles have been placed in the way to prevent this natural inclination and tendency from happening.
Please see prejudice, racism, social graces, socially unacceptable behavior, conservative thought, chauvinism, the battle of the sexes, homophobia, the pope, catholicism, war, sexually transmitted diseases, televangelism, interracial dating, terrorism, anti-terrorism, my god is bigger than your god, the fertile crescent, the tower of babel, and many more examples for further information and study.
Oh, and watch your head. It's bound to get bumpy.