US researchers surveyed 254 parents of youngsters aged 11 to 27 with gender dysphoria. Picture: Wikimedia Commons

London - Teenagers who question their biological gender often have friends who have become transgender, researchers found.

Twenty-one percent of teenagers and young adults who suddenly questioned their gender identity after puberty had one or more friend who "came out" as transgender around the same time, according to a study of their parents.

US researchers surveyed 254 parents of youngsters aged 11 to 27 with gender dysphoria – the distress of feeling you are a different gender to the one you were born with – that came on after puberty. In 86.5 percent of cases, the children were reportedly the second, third or fourth within their friendship group to question their gender.

Study author Dr Lisa Littman, from Brown University, in Providence, Rhode Island, said: "Of the parents who provided information about their child’s friendship group, about a third responded that more than half of the kids in the friendship group became transgender-identified."

One parent said their child had seen a "great increase in popularity" by identifying as transgender, adding: "Being trans is a gold star in the eyes of other teens."

One in five parents reported an increase in their child’s social media use around the same time as their gender dysphoria, with parents suspecting YouTube "transition" videos had influenced this.

The study says: "The description of cluster outbreaks of gender dysphoria occurring in pre-existing groups of friends and increased exposure to social media/internet preceding a child’s announcement of a transgender identity raises the possibility of social and peer contagion."

The study, published in the journal PLOS One, recruited parents from websites for those critical of the "transgendering" of young people. But 88 percent believed transgender people deserved the same rights as everyone else.