I created a questionnaire and spoke with 6 people about how they determine someone else in a public space to be a threat. Interviewees varied in cultural and economic background, and yet their answers to the question were resoundingly similar. They determine someone to be a threat based on "a feeling".
I learned that people use their intuition, or heuristics, which is grounded in their skewed exposures to people, cultures, and environments. Upon further research, I discovered that a person is likely to trust someone who reminds them of someone they've trusted in their past.
This means that an encounter with someone from a different culture might trigger feelings of suspicion.