The following is an excerpt from Chapter 6: Prioritizing Trustworthiness in the book Holy War for True Democracy.
People take shortcuts when deciding whom or what to trust. These shortcuts cause mistakes.
We trust the familiar more than the unfamiliar. This explains brand loyalty – why we always buy the same brand of underwear or we go to movies with actors we know. It is also why some people fear people with different clothing, language, hair, skin color or religion. Foreign exchange programs for high school students tend to increase trust across cultures partly because they increase familiarity.
We rely on indirect trust, even if it is wrong. Suppose our friend Tom seems to know everything from how to do home repair to the family history of everyone in town. We believe Tom is a trustworthy source of information because no one else we know comes close to his level of knowledge on those topics, and Tom always sounds so authoritative. Tom recently read a book recommended by his friend extolling the virtues of cabbage juice. We trust Tom and therefore start drinking lots of cabbage juice (at least until we start feeling sick).
High-technology marketing gurus describe this indirect string of information sources as the word-of-mouth influencing pyramid, first articulated by Regis McKenna.  Here is a picture adapted with permission from a personal branding book  that illustrates the influencing pyramid in action when someone who needs surgery tries to find the best surgeon.
The patient finds the surgeon by checking social media, where people with more knowledge than the patient make a recommendation. But where do the people on social media get their information? From people further down the pyramid. The pyramid narrows because fewer and fewer people have expert knowledge.
The same word-of-mouth information flow happens in the world of politics. People who do not follow politics closely will generally form opinions based on what they hear from people they trust who seem to have more expertise. Perhaps you trust the political information from cabbage-juice Tom!
There is also the halo effect,  where we trust a particular politician (e.g., Obama or Trump) completely because their rhetoric matches our beliefs. Thus, many liberals trusted Obama when he said, if you like your current health-care policy, you can keep it. Similarly, if you are conservative, you might truly believe Mexico will pay for that wall.
Therefore, we take shortcuts with trust that sometimes results in imperfect results.