Trust is an interesting concept. By the time you get to work in the morning, you may have chosen to trust or not trust a dozen people. When you turn on the weather channel, you are choosing to trust the meteorologist. When you leave your jewelry on your dressing table, you do so because you trust the cleaning person who will come in the afternoon. When you count your change at the deli, you are choosing to not trust the cashier. Even spending money requires trusting that the otherwise worthless rectangle of green material in your hand has value.

Trust is what keeps our society functioning. Evolutionarily speaking, we must trust to survive. But it can be a slippery thing. What makes us trust people? And more curiously, what makes us trust some people but not others?

According to the “experts”—sociologists, psychologists, economists, political scientists—trust is based on expectation. To the degree you believe you can expect a certain response from someone, you trust him. To the degree you believe he will reciprocate at some point in the future in some (often undefined) way, you trust him. Of course, past experience—with the person in question or with others—will affect that confidence, but in the here and now, certain behaviors and visual cues can also influence if and how much you trust someone:

1. Familiarity. The more contact you have with someone, the more information you collect about him or her. The more information you have, the more confident you can be in your expectations.

2. Resemblance. If someone looks, dresses, or acts like you, you’re more likely to believe his or her actions and reactions will be similar to your own. A 2002 study at a Canadian university showed that people are more likely to trust someone whose facial features resemble theirs.

3. Consistency. The more someone behaves with consistency, the better you’re able to establish patterns and form expectations.

4. Punctuality. If someone is regularly on time, it not only signals consistency, but also general conscientiousness toward other people.

5. Flexibility. Social-exchange theorists have found that people are more likely to trust someone who does not try to explicitly negotiate or force a binding agreement. (Think of the last car salesman you encountered.)

6. Discretion. The ability to keep a secret and exercise tact will always inspire trust.

7. Transparency. The flip side of discretion is transparency. We want someone to keep our secrets, but not her own. Self-disclosure builds trust.

8. Competence. In the workplace, nothing inspires trust more than getting the job done right.

9. Engagement. Trust is based on an understood reciprocity. If someone does not even appear to invest in you, he likely doesn’t have much to lose in betraying you.

10. Face Time. Part of engaging is an effort to make “face time.” A recent study showed that people in the workplace are more likely to trust team members with whom they interact in person more than those they work with via email or videoconference.

11. Facial hair. Another recent study in the Journal of Marketing Communications found that consumers trust pitchmen with beards more than those without. There are limits, however, to the beard-trustworthiness theory. Graphic designer Matt McInerney was only halfway kidding when he made a graphic spectrum of “The Trustworthiness of Beards.”

12. Eye contact (but not too much). This is perhaps the biggest behavioral indicator of trustworthiness. But the quality of the eye contact, observes psychologist Elaine Ducharme, also matters. Is it steely or warm? Too much eye contact can be unnerving.

13. Handshake (not too firm, not too soft). Any businessperson can tell you the importance of a firm handshake in building confidence. However, like eye contact, there is a middle ground. Too firm suggests aggression; too soft suggests passivity.

14. Posture. No one trusts a slouch. A straight back projects an image of strength and confidence.

Of course, while these behaviors and visual cues might inspire trust, they don’t guarantee trustworthiness.

As Ducharme wryly reminds, many psychopaths maintain excellent eye contact.