The scalp whorl, often called a crown, is a common feature on the human head where hair grows in a circular or spiral pattern. For most people, this pattern presents as a single swirl. However, a small percentage of the population exhibits two distinct swirls, a trait known as a double crown. This exploration delves into the scientific understanding of the double crown, its prevalence, and the genetics that govern this unique trait.
Defining the Scalp Whorl
A single scalp whorl is defined as the central point on the head where hair follicles align to cause the hair to grow in a circular, spiraling fashion. This swirl is typically located toward the back of the head, at the vertex, and is present in nearly every person. The hair within the whorl rotates outwards from this central point, which can sometimes appear as a slightly thinner spot on the scalp.
A double crown is characterized by the presence of two separate, distinct hair whorls on the scalp, most often situated near the vertex. These two whorls can spiral in the same direction, such as both clockwise, or they can rotate in opposite directions, with one clockwise and the other counter-clockwise.
The True Rarity of the Double Crown
The double crown is an uncommon variation of human hair morphology. Statistical data indicates that a double crown affects approximately 5% of the general population. This figure is in stark contrast to the near-universal presence of a single crown in almost 100% of people.
Some studies suggest that the prevalence of the double crown may vary across different ethnic groups. For instance, research indicates a higher frequency of double crowns in Afro-Caribbean populations compared to Caucasian populations, where the 5% figure is often cited.
Despite its lower prevalence, having two crowns is considered a normal variation of the human body, not a medical condition or an anomaly. The two whorls themselves do not cause any health issues, though the presence of two centers of rotation can sometimes make the hair appear thinner in those areas.
Genetics and Development of Whorl Patterns
The formation of hair whorls is an early event in human development, with the pattern becoming established between the 10th and 16th weeks of gestation. During this time, the rapid expansion of the fetal brain creates mechanical tension on the scalp skin. This tension, combined with the direction of hair follicle growth, helps determine the final spiral pattern of the whorl.
The trait is understood to have a strong genetic basis, following a pattern of polygenic inheritance, meaning multiple genes influence its expression. While the exact genes are complex and not completely identified, recent genome-wide association studies have pinpointed several genetic variants that likely influence hair whorl direction.
There is a long-debated link between the direction of a single hair whorl and handedness, with some observations suggesting a correlation between right-handedness and clockwise whorls. This link remains a scientific observation rather than a hard rule, and modern genetic analysis suggests the pathways for hair follicle growth and brain lateralization are distinct.
Dispelling Whorl-Related Myths
The distinctiveness of the double crown has led to various pieces of folklore and anecdotal beliefs across cultures. These misconceptions often attempt to link the presence of two whorls to personality traits, higher intelligence, or even temperament. Other common myths include an association with increased risk of balding or certain neurodevelopmental conditions.
No scientific evidence supports any correlation between the number of scalp whorls and any cognitive, behavioral, or psychological traits. The hair whorl is a purely physical characteristic, similar to eye or hair color, determined by genetics and developmental mechanics. The trait is merely a variation in the pattern of hair growth.