Brain pathways Many researchers focus on the brain to explain<unk> . 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<unk>  in these species, including vocalization. Electrical stimulation of the hypothalamus causes aggressive behavior and the hypothalamus has receptors that help determine<unk>  levels based on their interactions with serotonin and vasopressin. In rodents, activation of estrogen receptor-expressing neurons in the ventrolateral portion of the ventromedial hypothalamus (VMHvl) was found to be sufficient to initiate<unk>  in both males and females. Midbrain areas involved in<unk>  have direct connections with both the brainstem nuclei controlling these functions, and with structures such as the amygdala and prefrontal cortex. 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<unk>  (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<unk>  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<unk> . The role of the amygdala is less clear in primates and appears to depend more on situational context, with lesions leading to increases in either social affiliatory or aggressive responses. Amygdalotomy, which involves removing or destroying parts of the amygdala, has been performed on people to reduce their violent behaviour. The broad area of the cortex known as the prefrontal cortex (PFC) is crucial for self-control and inhibition of impulses, including inhibition of<unk>  and emotions. Reduced activity of the prefrontal cortex, in particular its medial and orbitofrontal portions, has been associated with violent/antisocial<unk> . In addition, reduced response inhibition has been found in violent offenders, compared to non-violent offenders. The role of the chemicals in the brain, particularly neurotransmitters, in<unk>  has also been examined. This varies depending on the pathway, the context and other factors such as gender. A deficit in serotonin has been theorized to have a primary role in causing impulsivity and<unk> . At least one epigenetic study supports this supposition. Nevertheless, low levels of serotonin transmission may explain a vulnerability to impulsiveness, potential<unk> , and may have an effect through interactions with other neurochemical systems. These include dopamine systems which are generally associated with attention and motivation toward rewards, and operate at various levels. Norepinephrine, also known as noradrenaline, may influence<unk>  responses both directly and indirectly through the hormonal system, the sympathetic nervous system or the central nervous system (including the brain). It appears to have different effects depending on the type of triggering stimulus, for example social isolation/rank versus shock/chemical agitation which appears not to have a linear relationship with<unk> . Similarly, GABA, although associated with inhibitory functions at many CNS synapses, sometimes shows a positive correlation with<unk> , including when potentiated by alcohol. The hormonal neuropeptides vasopressin and oxytocin play a key role in complex social behaviours in many mammals such as regulating attachment, social recognition, and<unk> . Vasopressin has been implicated in male-typical social behaviors which includes<unk> . Oxytocin may have a particular role in regulating female bonds with offspring and mates, including the use of protective<unk> . Initial studies in humans suggest some similar effects. In human, aggressive behavior has been associated with abnormalities in three principal regulatory systems in the body serotonin systems, catecholamine systems, and the hypothalamic–pituitary–adrenal axis. Abnormalities in these systems also are known to be induced by stress, either severe, acute stress or chronic low-grade stress Testosterone Early androgenization has an organizational effect on the developing brains of both males and females, making more neural circuits that control sexual behavior as well as intermale and interfemale<unk>  become more sensitive to testosterone. There are noticeable sex differences in<unk> . Testosterone is present to a lesser extent in females, who may be more sensitive to its effects. Animal studies have also indicated a link between incidents of<unk>  and the individual level of circulating testosterone. However, results in relation to primates, particularly humans, are less clear cut and are at best only suggestive of a positive association in some contexts. In humans, there is a seasonal variation in<unk>  associated with changes in testosterone. For example, in some primate species, such as rhesus monkeys and baboons, females are more likely to engage in fights around the time of ovulation as well as right before menstruation. If the results were the same in humans as they are in rhesus monkeys and baboons, then the increase in aggressive behaviors during ovulation is explained by the decline in estrogen levels. This makes normal testosterone levels more effective. Castrated mice and rats exhibit lower levels of<unk> . Males castrated as neonates exhibit low levels of<unk>  even when given testosterone throughout their development. Challenge hypothesis The challenge hypothesis outlines the dynamic relationship between plasma testosterone levels and<unk>  in mating contexts in many species. It proposes that testosterone is linked to<unk>  when it is beneficial for reproduction, such as in mate guarding and preventing the encroachment of intrasexual rivals. The challenge hypothesis predicts that seasonal patterns in testosterone levels in a species are a function of mating system (monogamy versus polygyny), paternal care, and male-male<unk>  in seasonal breeders. This pattern between testosterone and<unk>  was first observed in seasonally breeding birds, such as the song sparrow, where testosterone levels rise modestly with the onset of the breeding season to support basic reproductive functions. The hypothesis has been subsequently expanded and modified to predict relationships between testosterone and<unk>  in other species. For example, chimpanzees, which are continuous breeders, show significantly raised testosterone levels and aggressive male-male interactions when receptive and fertile females are present. Currently, no research has specified a relationship between the modified challenge hypothesis and human behavior, or the human nature of concealed ovulation, although some suggest it may apply. Effects on the nervous system Testosterone to Estradiol conversion Another line of research has focused on the proximate effects of circulating testosterone on the nervous system, as mediated by local metabolism within the brain. Testosterone can be metabolized to estradiol by the enzyme aromatase, or to dihydrotestosterone (DHT) by 5α-reductase. Aromatase is highly expressed in regions involved in the regulation of aggressive behavior, such as the amygdala and hypothalamus. In studies using genetic knockout techniques in inbred mice, male mice that lacked a functional aromatase enzyme displayed a marked reduction in<unk> . Long-term treatment with estradiol partially restored aggressive behavior, suggesting that the neural conversion of circulating testosterone to estradiol and its effect on estrogen receptors influences inter-male<unk> . In addition, two different estrogen receptors, ERα and ERβ, have been identified as having the ability to exert different effects on<unk>  in mice. However, the effect of estradiol appears to vary depending on the strain of mouse, and in some strains it reduces<unk>  during long days (16 h of light), while during short days (8 h of light) estradiol rapidly increases<unk> . Another hypothesis is that testosterone influences brain areas that control behavioral reactions. Studies in animal models indicate that<unk>  is affected by several interconnected cortical and subcortical structures within the so-called social behavior network. A study involving lesions and electrical-chemical stimulation in rodents and cats revealed that such a neural network consists of the medial amygdala, medial hypothalamus and periaqueductal grey (PAG), and it positively modulates reactive<unk> . Moreover, a study done in human subjects showed that prefrontal-amygdala connectivity is modulated by endogenous testosterone during social emotional behavior. In human studies, testosterone-aggression research has also focused on the role of the orbitofrontal cortex (OFC). This brain area is strongly associated with impulse control and self-regulation systems that integrate emotion, motivation, and cognition to guide context-appropriate behavior. Patients with localized lesions to the OFC engage in heightened reactive<unk> . Aggressive behavior may be regulated by testosterone via reduced medial OFC engagement following social provocation. When measuring participants' salivary testosterone, higher levels can predict subsequent aggressive behavioral reactions to unfairness faced during a task. Moreover, brain scanning with fMRI shows reduced activity in the medial OFC during such reactions. Such findings may suggest that a specific brain region, the OFC, is a key factor in understanding reactive<unk> . General associations with behavior Scientists have for a long time been interested in the relationship between testosterone and aggressive behavior. In most species, males are more aggressive than females. Castration of males usually has a pacifying effect on aggressive behavior in males. In humans, males engage in crime and especially violent crime more than females. The involvement in crime usually rises in the early teens to mid teens which happen
Aggression
Disinhibition
Dissociation 
Seclusion
Impaction
Perseveration
Derealization
Perspiration