Classical conditioning is a type of learning whereby an organism develops a reflexive response to a previously neutral stimulus after the neutral stimulus becomes paired with something that naturally invokes that response. A common example is an experiment in which Ivan Pavlov rang a bell while presenting meat powder to his dog. The meat powder naturally triggered salivation in the dog, while the bell was a neutral stimulus. Over time, the dog began salivating in response to the bell alone.
Classical conditioning produces only reflexive behaviors and does not apply to behaviors that organisms perform voluntarily, such as reading a book. Examples of involuntary behaviors that can arise due to classical conditioning include nausea, changes in appetite, pupil dilation and changes in heart rate. People commonly come to have intense emotional responses in the presence of certain smells or sounds. For example, a woman may feel joyful when she smells a cologne her husband frequently wears. The cologne, a previously neutral stimulus, comes to elicit positive emotions because of its frequent pairing with the presence of her husband.
Classical conditioning is involved in the development of phobias and taste aversions. A person who was never fearful of dogs in the past may develop a phobia of dogs after a dog bite because the sight of a dog is now associated with the fearful experience of being attacked. If a person becomes sick after eating a particular food, she may develop a taste aversion to that food and automatically become nauseous in the food's presence in the future.