Why do we trust whom we trust? Why do we refuse to trust sometimes? Experience (with the persons in questions) is a big part of it, but not always. And what does this mean for our relationships?
One source of trust is us belonging to a group (or tribe, or nation). Some inter-disciplinary researchers have established that we tend to be altruistic within these groups, and less so between the groups. This is called ‘parochial’. On average, we tend to make choices of benefit to our group (even when conflicting with the choice that benefits all groups), out of a sense of duty. People perceive that they got something from their nation or group, so have a duty to give back.
There are some benefits of organizing our trust in this way. It is more ‘efficient’ in the short term; agreements are easier to enforce in a group with limited membership.
But when you are looking for your significant other, parochialism isn’t the smartest thing. It segments the world. Selecting your best possible mate just within your segment won’t likely find you the best possible mate in the world.