Being able to look someone in the eye has, usually, meant that you have nothing to be ashamed of, while avoiding eye contact, in modern western culture, is generally thought of as being a bit shifty. Teachers and parents might once have reprimanded a child for averting his or her eyes by saying: "Look at me when I'm speaking." But is that all changing?
Certainly, most of us would expect to maintain eye contact in professional relationships - what would we think of a doctor who refused to meet our gaze? Yet, increasingly, there seem to be situations in which people report feeling awkward or embarrassed at direct eye contact.
Could this be a symptom of our urban life styles, where we are forced to live and work at close quarters with one another? In 2005, the Journal of Nonverbal Behaviour studied passengers on commuter trains in the United States. It found that commuters were much more likely to make eye contact with passers-by in suburban stations, but as the train neared stops in the city, eye contact was avoided.
Even in Britain, where looking is concerned, the overriding rule seems to be that it's "rude to stare". Analysts have pegged Britain as a "low-look culture" because people tend to find eye contact intrusive. The London Underground is the best-known example. At rush hour, when commuters are packed into trains like sardines, crushed up against one another, everyone is looking down, reading, gazing out the window at the tunnel walls - anything not to catch the eye of a fellow traveller.
In a way, this shouldn't be surprising. Since the 1960s, body positioning and eye contact have been considered a valuable part of "speaking". So just as we find the tinny sound of someone else's iPod headphones intrusive, so would we wish to avoid unsolicited communication with a stranger. What's more, as the meaning of looking somebody straight in the eye is culturally defined, it follows that we would want to avoid doing so in a environment where people from countless different cultures and traditions are thrown together.
To be stared at by a stranger causes uncertainty. What do they mean by it? Is it a challenge, or a greeting? Casual glances in busy urban areas are often misinterpreted as invading someone's personal space - any teenager who has grown up in a city will tell you that looking at someone "the wrong way" is taken as an invitation to a fight.
And just as eye contact has become a taboo in modern urban life, it has also become something of an illicit pleasure. A casual glance can be extremely flirtatious - so much so, that one London newspaper runs a column where people can leave messages asking out random strangers they glanced at on public transport. After all, it may be rude to stare, but deep down, isn't that exactly what we'd all love to do?