that have a moral value. These do not necessarily activate defensive systems; they are more or less pure forms of cognitive anxiety. Co-morbidity Anxiety disorders often occur with other mental health disorders, particularly major depressive disorder, bipolar disorder, eating disorders, or certain personality disorders. It also commonly occurs with personality traits such as neuroticism. This observed co-occurrence is partly due to genetic and environmental influences shared between these traits and anxiety. It is common for those with obsessive–compulsive disorder to experience anxiety. Anxiety is also commonly found in those who experience panic disorders, phobic anxiety disorders, severe stress, dissociative disorders, somatoform disorders, and some neurotic disorders. Anxiety has also been linked to the experience of intrusive thoughts. Studies have revealed that individuals who experience high levels of anxiety (also known as clinical anxiety) are highly vulnerable to the experience of intense intrusive thoughts or psychological disorders that are characterised by intrusive thoughts. Risk factors Anxiety disorders are partly genetic, with twin studies suggesting 30-40% genetic influence on individual differences in anxiety. Environmental factors are also important. Twin studies show that individual-specific environments have a large influence on anxiety, whereas shared environmental influences (environments that affect twins in the same way) operate during childhood but decline through adolescence. Specific measured 'environments' that have been associated with anxiety include child abuse, family history of mental health disorders, and poverty. Anxiety is also associated with drug use, including alcohol, caffeine, and benzodiazepines (which are often prescribed to treat anxiety). Neuroanatomy Neural circuitry involving the amygdala (which regulates emotions like anxiety and fear, stimulating the HPA axis and sympathetic nervous system) and hippocampus (which is implicated in emotional memory along with the amygdala) is thought to underlie anxiety. People who have anxiety tend to show high activity in response to emotional stimuli in the amygdala. Some writers believe that excessive anxiety can lead to an overpotentiation of the limbic system (which includes the amygdala and nucleus accumbens), giving increased future anxiety, but this does not appear to have been proven. Research upon adolescents who as infants had been highly apprehensive, vigilant, and fearful finds that their nucleus accumbens is more sensitive than that in other people when deciding to make an action that determined whether they received a reward. This suggests a link between circuits responsible for fear and also reward in anxious people. As researchers note, "a sense of 'responsibility', or self-agency, in