Diversity of Hair Types
To characterize the diversity of hair types worldwide and identify
their various properties, L’Oréal researchers have studied several
different parameters: the shape of the hair shaft, the structure
of the hair fibre components, the growth of the hair follicle and its
pigmentation. They have drawn on techniques from biology, chemistry
and biophysics, using optical and digital microscopic imaging.
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 colour. An evaluation of natural hair colour based on volunteers’ geographical origin, age and sex showed that the natural colour 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 colours are mainly limited to Northern and Eastern Europe, while Asian, Melanesian and African hair is characterised by darker colours 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).
For the first time, L’Oréal researchers have developed a global classification of hair based on eight categories ranging from very straight (class I) to very curly (class VIII) 
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 programmes. 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 characterised by both slow hair growth (280 +/- 50 μm per day regardless of sex or scalp area) and low hair density (161 +/- 50 hairs per cm2, with a greater number on the vertex)
• Chinese hair also has a low density (175 +/- 54 hairs per cm2 ) but grows very fast (411 +/- 53 μm per day)
• Caucasian hair grows at an intermediate rate of 367 +/- 56 μm per day and is very dense (226 +/- 73 hairs per cm2).
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 behaviour 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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 Porter CE, et al. The influence of African-American hair’s curl pattern on its mechanical properties. Int J Dermatol. 2005; 44 (SUPPL.1):14–5