SKINNY girls with blank expressions and seemingly little inclination to speak have fascinated American culture for so long that their proliferation now, in magazines like Us Weekly and In Touch and shows like “The Hills,” seems less a modern scourge than a historic inevitability. The mid-19th century witnessed the emergence of the aptly named fasting girls, women in their teens and early 20s whose silence and diminutive size stirred the interest of a public that believed they were spiritually extraordinary. Later they found analogues in figures like Edie Sedgwick, on whom so many fantasies  stylistic, sexual, psychological  were projected, and more recently in the phenomenon of the Olsen twins.

Who are the Olsens? What are the Olsens? Biographically speaking, they are the genetically fraternal but identical-looking sisters, Mary-Kate and Ashley, 22, who laid initial claim to our attention when they shared the role of Michelle Elizabeth Tanner in the 1980s and ’90s on the family comedy “Full House.”

Symbolically, they are harder to define because they defy the standard categories of American celebrity. They have acted, but acting is now just a peripheral part of their identity. They appear regularly in tabloids, yet they cannot fairly be included among the Parises and Nicoles, the Laurens and Heidis  the sisterhood of young women famous only for their professional apathy.

Image Mary-Kate Olsen, left, and Ashley Olsen at a Chanel show in Paris. Credit... Eric Ryan/Getty Images

The Olsens, as it happens, do quite a bit: they oversee 18 employees as co-presidents of the multimillion-dollar company Dualstar Entertainment Group, which distributes the direct-to-video movies they made when they were younger and, through licensing arrangements, produces furniture, rugs, lighting and cosmetics for girls 8 to 12. Through a licenser, the Olsens also turn out a line of bohemian women’s clothing called Elizabeth and James, and on their own they produce a more rarefied label, for women with the means to buy Chanel or Prada, called the Row. Most recently they have also written a book, “Influence” (Razorbill), that seeks to convey the essence of their creative vision, citing as sources of inspiration designers, fashion photographers and artists, among them Karl Lagerfeld, Terry Richardson and Richard Prince.