Defining social trust is a first step toward nurturing it
At some point in human evolution, humans learned to trust strangers with their lives. Before, social life meant living in a familiar tribe. One might interact with other groups, but that could involve violence as often as, say, trade. Humans trusted those they knew, despite occasional trust in strangers.
As societies grew, humans increased contact with strangers, especially in new urban environments. Somehow, humans learned to get along with people they do not know and will never know. Not every merchant from another country posed a threat. Nor was every traveller criminal.