Trump's biggest foray into politics before his election as president came as part of a baseless conspiracy that President Barack Obama was not born in the United States and was therefore ineligible to be president. As Trump continued to press the issue for years, he lied about a variety of things and made statements like this: "I have people that have been studying [Obama's birth certificate] and they cannot believe what they're finding ... I would like to have him show his birth certificate, and can I be honest with you, I hope he can. Because if he can't, if he can't, if he wasn't born in this country, which is a real possibility ... then he has pulled one of the great cons in the history of politics." (NBC)

There is no basis in fact for this conspiracy claim. When he finally admitted this was the case, he then falsely claimed that Hillary Clinton's campaign started the rumor. 

Meanwhile, his nominee for National Security Adviser, Michael Flynn, is also susceptible to belief in conspiracies. Flynn believes Islamic law, known as Shariah, is spreading in the United States. This is not true. Such assertions by Flynn became common enough that his subordinates at the Defense Intelligence Agency called them "Flynn facts." (Source: The New York Times) 

Flynn's son, who served as his chief of staff and also had contact during the campaign with Trump, also buys into and spreads conspiracy theories. His Twitter and Facebook accounts have contained conspiracies and racially insensitive statements. One theory he pushed was that Trump's opponent Marco Rubio, a Florida senator, was gay and abused cocaine. (CNN) He also pushed the false conspiracy theory that Hillary Clinton was part of a pedophilia ring running out of a Washington, D.C. pizzeria, which lead an armed vigilante to show up with an assault rifle. Flynn's son resigned from Trump's transition team shortly before he was fired for being a distraction. (CBS)

See also Assaults on Facts and False Statements.

Timeline

Analysis

What makes people believe in conspiracy theories? They flare up during times of uncertainty and fear, experts say. Terrorist strikes, financial crises, high-profile deaths, and natural disasters can trigger people's desire for control. Trying to make sense of things "leads them to connect dots that aren’t necessarily connected in reality,” according to Jan-Willem van Prooijen, associate professor in social and organizational psychology at VU University Amsterdam, who has studied the phenomenon for years.