Once a macho biker name, Harley is now showing its softer side, given to over 1100 girls last year, which is thrice the number of boy Harleys. In the UK, Harley is still much more male—it's currently Number 45 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. With upcoming DC Comics movie Suicide Squad on the horizon for 2016, Ms. Quinn could impact this name's rise even more. 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.