What is a common cause of ingrown hairs after shaving?

Ingrown hairs commonly occur when hair is cut too short during shaving and subsequently curls back and grows into the skin. This situation often arises when individuals shave against the natural direction of hair growth or use improper techniques. When the hair grows back inward rather than outward, it can lead to inflammation or discomfort, resulting in ingrown hairs.

While using dull blades can contribute to irritation, leading to poorer shaving results, it is not the primary cause of hair curling back into the skin. Similarly, using excessive shaving cream may affect the glide of the razor but does not inherently cause hair to grow inward. Not rinsing the razor properly can lead to clogged blades and irritation as well, but this again does not directly induce hairs to grow back into the skin. The most direct cause of ingrown hairs is the behavior of the hair itself post-shaving, making the option about hair curling back into the skin the correct choice.