Hair crowns, or hair whorls, are patterns of hair growth that can be seen as a "spiral" on the back of the head. Double hair crowns appear as two spirals. In a 1975 genetic study by Wunderlich and Hereema, 1.5 percent of the population was estimated to have double crowns, according to John H. McDonald from the University of Delaware.
While double crowns are rare, there is no evidence to suggest that anything unusual "causes" them. Genetics determine single or double crowns. Hair whorls can grow in spirals in clockwise or counterclockwise formations, notes Amar J. S. Klar for Genetics.
In the same Wunderlich and Hereema study, an estimated 94 percent of newborns in 1975 had clockwise whorls. In 2004, Klar published an observed correlation between clockwise hair whorls and right-hand dominance. Similarly, people with counterclockwise whorls favored left-handedness. As noted in the study, it may be a coincidence, as right-handedness, like a clockwise hair whorl, are more commonly observed in the general population. Additionally, throughout studies conducted from 1975 to 2009, it was noted that most women do not have crowns as evident as men; instead, women have a "diffused" pattern of hair growth. It is more difficult to identify the crown in women with long or curly hair. Because of this, geneticists have not been able to pinpoint a gene that "causes" double crowns, but assert that the parent's hair whorls generally mirror their child's, notes McDonald.