The term instinct is commonly thought of as describing behavior that is pre-programmed and requires no thought. An instinctive response happens automatically and is the same across all individuals of a species. While this seems like a clearly defined category, not all behavior is so cut and dry. Many behaviors have both genetically pre-programmed and learned aspects. The degree to which certain behaviors, especially in humans, are pre-determined is often a subject of intense debate.
When looking at instinctive behaviors, we must first understand the terminology used to describe such behaviors, and then we must consider some of the concerns involved in studying them. Any thing or event that triggers a behavior is called a key stimulus (KS). Key stimuli produce fixed action patterns (FAP) through innate releasing mechanisms (IRM). Several KS may be needed to trigger an FAP and different degrees of a KS can result in different strength FAP response. Different classes of sensory receptor cells are important in the receiving of different types of KS. The associated neural networks of these different sensory cells help to integrate the signal from many receptors to determine the degree of the KS and thereby produce an appropriate level of response. Many of these responses are determined by carefully regulated internal messenger molecules called hormones. The endocrine system, which is responsible for the production and transport of hormones throughout the body, is made up of many secretory glands that produce hormones and release them for transport to target organs. In vertebrates, neural control of this system is funneled through the hypothalamus to the anterior and posterior pituitary gland.
Whether the behavioral response to a given KS is learned, genetic, or both is the subject of study in the field of behavioral genetics. Researchers use techniques such as inbreeding and knockout studies to attempt to separate learning and environment from genetic determination of behavioral traits.