I don't know....genetics? His sons would seem to prove this theory.
Rex Tillerson thinks so: https://youtu.be/dgI6XiYnVlk
101 reasons why hs is: https://www.amazon.com/Indisputable-Facts-Proving-Donald-Trump-ebook/dp/B07DWFH672'
Because he doesn't comprehend basic science and presentations of science: https://www.vanityfair.com/news/2020/04/donald-trump-lysol-coronavirus-warning
Some good reasons here: https://www.washingtonblade.com/2020/03/25/trumps-epic-fail-a-moron-is-president/
His aids think he is: https://www.politico.com/story/2018/09/04/trumps-insults-idiot-woodward-806455
• Defense Secretary Jim Mattis said Trump had the understanding of “a fifth- or sixth-grader,” according to accounts of Woodward’s book that were published by The Washington Post. The comments came after a National Security Council meeting on Jan. 19 in which Trump questioned why the government was using resources to maintain a U.S. military presence on the Korean Peninsula. Mattis told Trump that presence was necessary “in order to prevent World War III.”
• White House chief of staff John Kelly called Trump “an idiot” and said he thought the president was “unhinged,” The Post also reported. NBC News first reported in May that Kelly had referred to the president as an idiot multiple times, in addition to making several remarks “insulting the president’s intelligence and casting himself as the savior of the country.” Kelly has denied that he ever called Trump an idiot.
• Omarosa Manigault Newman, who was the highest-ranking African-American staffer in the West Wing, claimed in a book published earlier this summer that Trump is a “racist, misogynist and bigot.” In her book, “Unhinged,” Manigault Newman said she witnessed Trump use racial epithets while describing presidential counselor Kellyanne Conway’s husband, George Conway, who is half Filipino.
• Earlier this year, Michael Wolff detailed in his book “Fire and Fury” that a number of individuals in the Trump administration insulted the president’s intelligence. Both Treasury Secretary Steven Mnuchin and former chief of staff Reince Priebus called Trump an “idiot,” Wolff wrote. Former economic adviser Gary Cohn said Trump was “dumb as shit,” and former national security adviser H.R. McMaster said the president was a “dope,” according to the book.