Genetic or Biological Contributors to Porosity
Genetics and hair type (straight, wavy, or curly) contribute to how tightly the cuticle layer adheres. Some people have a circular hair shaft, which is optimal for cuticle scales to lie flat and overlap one another, resulting in low porosity. Other types of hair are more elliptical or even flat and ribbon-like. This geometry doesn't allow for all of the cuticle scales to lie flat and overlap one another along the axis of the hair, especially at the outer edges of the hair strand. This creates areas of discontinuity in the cuticle layer, which adds porosity to the hair.
In a similar fashion, curly hair has a tendency to be naturally higher in porosity than straight hair. This is because the spirals in the helical configuration of the curls create areas where individual cuticle scales are raised slightly away from the longitudinal axis of the hair. The curlier the hair is, the more breaks you have in the smoothness of the surface, so the porosity is invariably increased.
External Contributors to Porosity
Environment: Exposure to UV rays for prolonged periods can fuse cuticle scales together, which inevitably leads to further damage of the cuticle layer. It is a good idea to cover your hair when in the sun or use products which contain sunscreen agents.
Chemical Processing: Perms, relaxers, and coloring processes all require the cuticle to first be opened via application of an alkaline solution. This allows the chemicals to access the interior of the hair shaft in order to make permanent changes to the structure of the proteins that are the building blocks of the hair. All of these processes are capable of doing permanent damage to the cuticle layer. This damage builds up with repeat use of the chemical process. Bleaching is the most damaging process, followed by perming and relaxing, with most permanent coloring processes being the mildest.
Heat treatments: Heat from a blow dryer, flat iron, curling iron, or hot curlers can all cause irreparable damage both to the cuticle and the cortex of the hair. These tools can heat water inside the hair past the boiling point and cause the hair to rupture from the inside out. It is not difficult to see how this could increase porosity.