What is the role of trust in ordinary social workings? I would say that a fairly high level of trust is simply mandatory in any social group, from a family to a workplace to a full society. Lacking trust, each agent is forced into a kind of Hobbesian calculation about the behavior of those around him or her, watching for covert strategies in which the other is trying to take advantage of oneself. The cost of self-protection is impossibly high in a zero-trust society. Gated communities don't help. We would need to have gated and solitary lives. Even our brothers and sisters, spouses, and offspring would have to be watched suspiciously. We would live like Howard Hughes at the end of his life.
To begin, what is trust? It is a condition of reliance on the statements, assurances, and basic good behavior of others. The status of commitments over time is essential to trust. We need to consider whether we can trust a neighbor who has promised to return a lawn mower -- will he keep his promise? Can we trust the car park attendant not to take the iPod from the glove compartment? Can we trust the phone company to not add hidden fees to our bill in a corporate decision that they won't be noticed by most consumers?
It is sort of a commonplace in moral philosophy that you can't trust a pure egoist or an act utilitarian. The reason is simple: trust means reliance on the correct behavior of other agents even when there is an opportunity for gain in incorrect behavior and the probability of detection and sanctions is low. The egoist will reason on the basis of the advantage he/she anticipates and will discount the low likelihood of sanction. But likewise, the act utilitarian will add up all the utilities created by "correct action" and "incorrect action", and will be bound to choose the action with the greater utility. The fact of an existing promise or other obligation will not change the calculation. So the act utilitarian cannot be trusted to honor his promises and obligations, no matter what.
Standards of "correct behavior" are difficult to articulate precisely, but here's a start: telling the truth, keeping promises and assurances when they come into play, acting according to generally shared rules of professional and social ethics, and respecting the rights of others. We sometimes describe people and organizations whose behavior conforms to these sorts of characteristics as possessing "integrity".
In general, agents whose behavior is governed solely by calculation of consequences cannot be trusted, since occasions requiring trust are precisely those in which we need to rely on others to do the right thing in spite of consequences that would favor doing the wrong thing. (For example, taking the iPod in circumstances where there are dozens of attendants and the theft cannot be attributed to one person; keeping the lawnmower if the owner is in a state of rapid-onset dementia; adding the phony charges in a business environment where it can be predicted with confidence that only 5% of customers will notice and the penalties are trivial.)
So there are two basic models of action that people can choose: consequentialist and "constrained by obligations" (deontological). The first approach is opportunistic and myopic; the other reflects integrity and the validity of long-term obligations.
But here we have a problem. The most ordinary social transactions become almost impossible in a no-trust environment. If I can't trust my bank to hold my savings honestly, or my employer to keep its commitments about my retirement accounts, or the passenger on the seat next to me on a long airplane flight to not go through my briefcase if I drift off to sleep -- then I am forced into a condition of exhausting, sleepless vigilance. And, of course, we do generally trust in these circumstances.
But it is an interesting problem for research to consider whether different societies and groups elicit and sustain different levels of trust in ordinary life, and what the institutional factors are that affect this outcome. Is there a higher level of trust in Bloomington, Illinois than Chicago or Houston? Is trust a feature of the learning environment through which people gain their social psychologies? Are there institutional features that encourage or discourage dispositions towards trust? And what are the compensating mechanisms through which social interactions proceed in a low-trust environment? Is that where "trust but verify" comes in?