The need for people to feel connected runs deep. According to a study conducted at Purdue University, even the gaze of a stranger makes a difference. When strangers pass you by without acknowledging you, you feel more disconnected. And it hurts.

The study looked at traffic along a well-traveled path on campus. A research assistant walked along the path and either met a passing person's eyes, met their eyes and smiled or looked right past the person, essentially ignoring them. The person on the path was then immediately interviewed and asked how disconnected they felt right now.

Even being ignored by a group that they want nothing to do with can make people feel left out.

People who had gotten eye contact from the research assistant, with or without a smile, felt less disconnected than those who had been ignored.

Feeling connected doesn't only make you happier, it can add years to your life. It can even help you quit smoking. But it's not how rich or poor your social network actually is that seems to make the most difference, it's how well-connected you think you are that seems to be the driving force. And according to this study, people need to feel connected to the strangers in their life so much that being ignored, even by a stranger, hurts.

In fact, the need to belong is so strong and runs so deep that it may not make a difference who you belong to as long as you do belong. Other research has found that even being ignored by a group that they want nothing to do with – like the Ku Klux Klan – can make people feel left out, so it's not totally surprising that being ignored by a stranger would have a similar effect.

The study authors, from Purdue, Universidad Nacional de Mar del Plata in Argentina, and Ohio State University, see their results as underscoring the need for social civility. Think about how you felt the last time someone looked at you as if you weren't there. The air gaze phenomenon is also likely to be familiar to members of ethnic groups who experience prejudice.

An article on the study appears in the journal, Psychological Science.