Valerie Biden Owens, the former vice president’s younger sister, says that one lasting impact of his childhood stutter is that it has given him more empathy and compassion for others’ trials, and it uniquely equips him to handle Trump’s taunts.

“Trump is a bully, and Joe has been standing up to bullies his entire life,” Owens said in an interview. “Joe’s stuttering, I think, is one of the principal reasons — a major, major, major reason — that he is the good and compassionate and kind man that he is.”

About 3 million Americans suffer from the speech impediment of stuttering, marked by involuntary repetition of sounds, syllables or words. According to the National Institute on Deafness and other Communication Disorders, most children outgrow their stutter, but for 25% of them, stuttering is a lifelong challenge.

Biden has overcome the serious stutter of his youth, but remnants of it resurface on occasions such as when he is very tired, he said in a 2016 speech. Experts on stuttering who follow him closely say they have noticed it on several occasions during the campaign, such as an interview on “The View” when he addressed complaints about his tendency to touch and hug women while campaigning, and an April speech in Pittsburgh launching his campaign, when he struggled with words.