young birds have been experimentally reared in devices that prevent them from moving their wings until they reached the age at which their cohorts were flying. these birds flew immediately and normally when released, showing that their improvement resulted from neuromuscular maturation and not true learning. in evolution imprinting provides one example of instinct. this complex response may involve visual, auditory, and olfactory cues in the environment surrounding an organism. in some cases, imprinting attaches an offspring to its parent, which is a reproductive benefit to offspring survival. if an offspring has attachment to a parent, it is more likely to stay nearby under parental protection. attached offspring are also more likely to learn from a parental figure when interacting closely. (reproductive benefits are a driving force behind natural selection.) environment is an important factor in the evolution of innate behaviour. a hypothesis of michael mccollough, a positive psychologist, explains that environment plays a key role in human behaviours such as forgiveness and revenge. this hypothesis theorizes that various social environments cause either forgiveness or revenge to prevail. mccollough relates his theory to game theory. in a tit-for-tat strategy, cooperation and retaliation are comparable to forgiveness and revenge. the choice between the two can be beneficial or detrimental, depending on what the partner-organism chooses. though this psychological example of game theory does not have such directly measurable results, it provides an interesting theory of unique thought. from a more biological standpoint, the brain's limbic system operates as the main control-area for response to certain stimuli, including a variety of instinctual behaviour. the limbic system processes external stimuli related to emotions, social activity, and motivation, which propagates a behavioural response. some behaviours include maternal care, aggression, defense, and social hierarchy. these behaviours are influenced by sensory input — sight, sound, touch, and smell. within the circuitry of the limbic system, there are various places where evolution could have taken place, or could take place in the future. for example, many rodents have receptors in the vomeronasal organ that respond explicitly to predator stimuli that specifically relate to that individual species of rodent. the reception of a predatory stimulus usually creates a response of defense or fear. mating in rats follows a similar mechanism. the vomeronasal organ and the main olfactory epithelium, together called the olfactory system, detect pheromones from the opposite sex. these signals then travel to the medial amygdala, which disperses the signal to a variety of brain parts. the pathways involved with innate circuitry are extremely specialized and specific. various organs and sensory receptors play parts in this complex process. instinct is a phenomenon that can be investigated from a multitude of angles: genetics, limbic system, nervous pathways, and environment. researchers can study levels of instincts, from molecular to groups of individuals. extremely specialized systems have evolved, resulting in individuals which exhibit behaviours without learning them.