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Fear is a negative emotion elicited by potential or actual threats. Often laboratory studies with rats are conducted to examine the acquisition and extinction of conditioned fear responses. In 2004, researchers conditioned rats (rattus norvegicus) to fear a certain stimulus, through electric shock. The researchers were able to then cause an extinction of this conditioned fear, to a point that no medications or drugs were able to further aid in the extinction process. However the rats did show signs of avoidance learning, not fear, but simply avoiding the area that brought pain to the tests rats. The avoidance learning of rats is seen as a conditioned response, and therefore the behavior can be unconditioned, as supported by the earlier research. Species-specific defense reactions (SSDRs) or avoidance learning in nature is the specific tendencies to avoid certain threats or stimuli, it is how animals survive in the wild. Humans and animals both share these species-specific defense reactions, such as the flight, fight, which also include pseudo-aggression, fake or intimidating aggression, freeze response to threats, which is controlled by the sympathetic nervous system. These SSDRs are learned very quickly through social interactions between others of the same species, other species, and interaction with the environment. These acquired set of reactions or responses are not easily forgotten. The animal that survives is the animal that already knows what to fear and how to avoid this threat. An example in humans is the reaction to the sight of a snake, many jump backwards before cognitively realizing what they are jumping away from, and in some cases it is a stick rather than a snake.

As with many functions of the brain, there are various regions of the brain involved in deciphering fear in humans and other nonhuman species The amygdala communicates both directions between the prefrontal cortex, hypothalamus, the sensory cortex, the hippocampus, thalamus, septum, and the  brainstem. The amygdala plays an important role in SSDR, such as the ventral amygdalofugal, which is essential for associative learning, and SSDRs are learned through interaction with the environment and others of the same species. An emotional response is created only after the signals have been relayed between the different regions of the brain, and activating the sympathetic nervous systems; which controls the flight, fight, freeze, fright, and faint response Often a damaged amygdala can cause impairment in the recognition of fear. This impairment can cause different species to lack the sensation of fear, and often can become overly confident, confronting larger peers, or walking up to predatory creatures.

Robert C. Bolles (1970), a researcher at University of Washington, wanted to understand species-specific defense reactions and avoidance learning among animals more, but found that the theories of avoidance learning and the tools that were used to measure this tendency were out of touch with the natural world. Dr. Bolles theorized the species -specific defense reaction (SSDR) There are three forms of SSDRs: flight, fight (pseudo-aggression), or freeze. Even domesticated animals have SSDRs, and in those moments its seen that animals revert to atavistic standards and become "wild" again. Dr. Bolles states that responses are often dependent on the reinforcement of a safety signal, and not the aversive conditioned stimuli. This safety signal can be a source of feedback or even stimulus change. Intrinsic feedback or information coming from within, muscle twitches, increased heart rate, is seen to be more important in SSRDs than extrinsic feedback, stimuli that comes from the external environment. Dr. Bolles found that most creatures have some intrinsic set of fears, to help assure survival of the species. Rats will run away from any shocking event, and pigeons (columbidae) will flap their wings harder when threatened, the wing flapping in pigeons and the scattered running of rats are considered a species-specific defense reaction or behavior. Bolles believes that SSDR are conditioned through pavlovian conditioning, and not operant conditioning; SSDR arise from the association between the environmental stimuli and adverse events. Michael S. Fanselow conducted an experiment, to test some specific defense reactions, he observed that rats in two different shock situations responded differently, on based on instinct or defensive topography, rather than contextual information

Species specific defense responses are created out of fear, and are essential for survival Rats that lack the gene stathmin show no avoidance learning, or a lack of fear, and will often walk directly up to cats and be eaten. Animals use these SSDR to continue living, to help increase their chance of fitness, by surviving long enough to procreate. Humans and animals alike have created fear to know what should avoided, and this fear can be learned through association with others in the community, or learned through personal experience with a creature, species, or situations that should be avoided. SSDRs are an evolutionary adaptation that has been seen in many species throughout the world including rats, chimpanzees (pan troglodytes), prairie dogs (cynomys), and even humans (homo sapiens), an adaptation created to help individual creatures survive in a hostile world.