But despite what pop culture may tell us, decades’ worth of attempts to decode the signals of trustworthiness — sought in everything from facial expression to voice to handwriting — have proven virtually useless, and the last five years of research have rendered previous assertions about certain nonverbal cues wrong. (No, a sideways glance doesn’t automatically indicate that the person is lying to you.) As DeSteno wryly observes, “If polygraphs were foolproof, we wouldn’t need juries.” He explains what makes measures of trust especially complicated:

Unlike many forms of communication, issues of trust are often characterized by a competition or battle…. It’s not always an adaptive strategy to be an open book to others, or even to ourselves. Consequently, trying to discern if someone can be trusted is fundamentally different from trying to assess characteristics like mathematical ability. … Deciding to be trustworthy depends on the momentary balance between competing mental forces pushing us in opposite directions, and being able to predict which of those forces is going to prevail in any one instance is a complicated business.

Trust is important for survival.

Knowing who to trust is among the most important decisions in our lives.

DeSteno, who has previously studied the osmosis of good and evil in all of us and the psychology of compassion and resilience, argues that matters of trust occupy an enormous amount of our mental energies and influence, directly or indirectly, practically every aspect of our everyday lives. But trust is a wholly different animal from the majority of our mental concerns. DeSteno writes:

Unlike many other puzzles we confront, questions of trust don’t just involve attempting to grasp and analyze a perplexing concept. They all share another characteristic: risk. So while it’s true that we turn our attention to many complex problems throughout our lives, finding the answers to most doesn’t usually involve navigating the treacherous landscape of our own and others’ competing desires.

[…]

Trust implies a seeming unknowable — a bet of sorts, if you will. At its base is a delicate problem centered on the balance between two dynamic and often opposing desires — a desire for someone else to meet your needs and his desire to meet his own.

I like the concept that trust boils down to risk.

Can I risk this person violating my trust in the worst case scenario?

Or put another way:  Do I choose to believe the universe will reward me for taking this risk, even if this particular person fails my trust?

Well said. Let's try another permutation:

Given that this person is likely to fail my trust, will the outcome still be something I can live with?

Both are healthier and wiser than my fantasy:  "Will my trust create trustworthiness in that individual?"  

I think people do learn to trust from each other, but I also think almost every person has something they would violate trust for, and therefore you need to build more fault tolerance into your system. ;)