Once a macho biker name, Harley is now showing its softer side, given to over 1400 girls last year, which is more than thrice the number of boy Harleys. In the UK, Harley is still much more male—it's currently Number 52 there.
Along with Harper and Hadley, we think it will climb even further. In England, it's associated with Harley Street, home to some of London's most prominent medical specialists.
Harley came over to the girls' side in 1991, long after being the name of the Joker's female assistant in the Batman universe, Harley Quinn, "born" Harleen Quinsel. DC Comics movie Suicide Squad from 2016 was probably part of the reason for the slight bump in the number of girls named Harley. Harley Cooper was a character who was introduced on the soap The Guiding Light in 1987—that Harley was named after her father's favorite motorcycle.