fear, frustration, anger, stress, dominance or pleasure (proximate causes). Sometimes aggressive behavior serves as a stress relief or a subjective feeling Aggression is behavior aimed at opposing or attacking something or someone. Though often done with the intent to cause harm, some might channel it into creative and practical outlets. It may occur either reactively or without provocation. In humans, aggression can be caused by various triggers. For example, aggression may result from built-up frustration due to blocked goals or perceived disrespec Aggression is behavior aimed at opposing or attacking something or someone. Though often done with the intent to cause harm, some might channel it into creative and practical outlets. It may occur either reactively or without provocation. In humans, aggression can be caused by various triggers. For example, aggression may result from built-up frustration due to blocked goals or perceived disrespect. Human aggression can be classified into direct and indirect aggression; while the former is characterized by physical or verbal behavior intended to cause harm to someone, the latter is characterized by behavior intended to harm the social relations of an individual or group. In definitions commonly used in the social sciences and behavioral sciences, aggression is an action or response by an individual that delivers something unpleasant to another person. Some definitions include that the individual must intend to harm another person. In an interdisciplinary perspective, aggression is regarded as "an ensemble of mechanisms formed during the course of evolution in order to assert oneself, relatives, or friends against others, to gain or to defend resources (ultimate causes) by harmful damaging means. These mechanisms are often motivated by emotions such as fear, frustration, anger, stress, dominance or pleasure (proximate causes). Sometimes aggressive behavior serves as a stress relief or a subjective feeling of power." Pr Sometimes aggressive behavior serves as a stress relief or a subjective feeling of power." Predatory or defensive behavior between members of different species may not be considered aggression in the same sense. Various factors limit the escalation of aggression, including communicative displays, conventions, and routines. In addition, following aggressive incidents, various forms of A cognitive approach to this relationship puts aggression in the broader context of inconsistency reduction, and proposes that aggressive behavior is caused by an inconsistency between a desired, or expected, situation and the actually perceived situation (e.g., "frustration"), and functions to forcefully manipulate the perception into matching the expected situation. In this approach, when the inconsistency between perception and expectancy is small, learning as a result of curiosity reduces inconsistency by updating expectancy to match perception. If the inconsistency is larger, fear or aggressive behavior may be employed to alter the perception in order to make it match expectancy, depending on the size of the inconsistency as well as the specific context. Uninhibited fear results in fleeing, thereby removing the inconsistent stimulus from the perceptual field and resolving the inconsistency. In some cases, thwarted escape may trigger aggressive behavior in an attempt to remove the thwarting stimulus. == Evolutionary explanations == Like many behaviors, aggression can be examined in terms of its ability to help an animal itself survive and reproduce, or alternatively to risk survival and reproduction. Numerous circuits within both neocortical and subcortical structures play a central role in controlling aggressive behavior, depending on the species, and the exact role of pathways may vary depending on the type of trigger or intention. In mammals, the hypothalamus and periaqueductal gray of the midbrain are critical areas, as shown in studies on cats, rats, and monkeys. These brain areas control the expression of both behavioral and autonomic components of aggression in these species, including vocalization. Stimulation of the amygdala results in augmented aggressive behavior in hamsters, while lesions of an evolutionarily homologous area in the lizard greatly reduce competitive drive and aggression (Bauman et al. 2006). In rhesus monkeys, neonatal lesions in the amygdala or hippocampus results in reduced expression of social dominance, related to the regulation of aggression and fear. Several experiments in attack-primed Syrian golden hamsters, for example, support the claim of circuitry within the amygdala being involved in control of aggression. The HPA axis is related to the general fight-or-flight response or acute stress reaction, and the role of catecholamines such as epinephrine, popularly known as adrenaline. === Pheromones === In many animals, aggression can be linked to pheromones released between conspecifics. In mice, major urinary proteins (Mups) have been demonstrated to promote innate aggressive behavior in males, and can be mediated by neuromodulatory systems. Mups activate olfactory sensory neurons in the vomeronasal organ (VNO), a subsystem of the nose known to detect pheromones via specific sensory receptors, of mice and rats. At least one study has found a possible epigenetic signature (i.e., decreased methylation at a specific CpG site on the promoter region) of the serotonin receptor 5-HT3a that is associated with maternal aggression among human subjects. Mice with experimentally elevated sensitivity to oxidative stress (through inhibition of copper-zinc superoxide dismutase, SOD1 activity) were tested for aggressive behavior. Males completely deficient in SOD1 were found to be more aggressive than both wild-type males and males that express 50% of this antioxidant enzyme. They were also faster to attack another male. The causal connection between SOD1 deficiency and increased aggression is not yet understood.