In humans, shyness is a feeling of insecurity that certain people experience while being among others, talking with others, asking favors of others, etc. In zoology, shy generally means "tends to avoid human beings".

Shyness is most likely to occur in an unfamiliar situation. Since many shy people thus avoid these situations in order to avoid feeling uncomfortable and inept, the situation remains unfamiliar and the shyness perpetuates itself. However, the initial cause of shyness can vary. Sometimes it seems to originate after the subject has experienced a physical anxiety reaction; at other times, shyness seems to develop first and then later causes physical symptoms of anxiety.

Scientists have located some genetic data that supports the hypothesis that shyness is at least partially genetic. However, it has also been proven that extroversion and introversion have much to do with the environment in which a child was raised, and not just its genetic makeup. A child who is shy toward strangers may eventually lose this trait as he grows up, or retain some tendency toward shyness.

Shy people do not necessarily feel the same degree of shyness toward all others. For instance, a heterosexual may be shy toward someone of the opposite sex, or an actor may be loud and bold onstage but shy in an interview.

Shy people tend to perceive their own shyness as a negative trait and many people are uneasy with shyness, especially in cultures that value individuality and taking charge. On the other hand, many shy people are perceived to be good listeners and are more likely to think before they speak. Furthermore, boldness, the opposite of shyness, may cause its own problems, such as impertinence or inappropriate behavior. This also applies to feigned boldness as a compensation for shyness.

Shyness is not directly related to introversion. Introverts choose to avoid social situations because they derive no reward from them, and may find the extra sensory input overwhelming. Shy people fear such situations and feel that they have to avoid them.