Fifty percent of the world’s population has dark to very dark brown hair
As with skin, the diversity of hair types worldwide is reflected in a continuum of color. An evaluation of natural hair color based on volunteers’ geographical origin, age and sex showed that the natural color of over 80% of the world’s population ranges  from black to light brown, and almost 50% of the population has dark or very dark brown hair. Blonde colors are mainly limited to Northern and Eastern Europe, while Asian, Melanesian and African hair is characterized by darker colors and less diversity.
From very straight to very curly hair
Hair shaft shape ranges from “straight with a circular cross-section” to “kinky with a highly elliptical cross-section and areas of torsion”. While shape is not specific to a given ethnic group, Asian hair is usually straight with a round cross-section, African hair tends to be twisted with a flattened cross-section and European hair is somewhere in between .
However, in order to move away from the three conventional ethnic groups
(African, Asian and Caucasian) and take into account the complex biological diversity created by extensive multiethnicity, L’Oréal researchers have established four hair shape descriptors: Curve diameter (CD), Curl index (i), Number of waves (w), and The Number of twists (t).
Human hair shape is programmed from the bulb
The bulb of curly hair is curved, while that of straight hair is straight in shape. L’Oréal biologists also found that the shape of the bulb is linked to asymmetry in cell differentiation programs. The curve of the hair is created by an internal mechanical force .
The speed at which hair grows depends on its shape
Hair growth measurements  using phototrichograms found that:
• African hair is characterized by both slow hair growth and low hair density
• Chinese hair also has a low density but grows very fast
• Caucasian hair grows at an intermediate rate per day and is very dense
The relationship between hair growth rate and hair morphology  was identified based on the observation that thick hair strands (types I and II) grow quickly, whereas thin hair strands grow more slowly.
Curly hair is more fragile than straight hair
The physical differences measured when comparing several African American hair samples with different curl patterns suggest that the curl pattern influences the behavior of the hair, and in particular its resistance to mechanical stress  . The curlier the hair, the smaller the curve diameter and, given that very curly hair stretches less under stress, it is more likely to break.
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