costs associated with ADHD. The estimated cost of illness for a child with ADHD is $14,576 (in 2005 dollars) annually. Prevalence of ADHD among prison populations is significantly higher than that of the normal population. In both adults and children, ADHD has a high rate of comorbidity with other mental health disorders such as learning disability, conduct disorder, anxiety disorder, major depressive disorder, bipolar disorder, and substance use disorders. The precise genetic and environmental factors contributing to ADHD are relatively unknown, but endophenotypes offer a potential middle ground between genes and symptoms. ADHD is commonly linked to "core" deficits involving "executive function," "delay aversion," or "activation/arousal" theories that attempt to explain ADHD through its symptomology. Endophenotypes, on the other hand, purport to identify potential behavioral markers that correlate with specific genetic etiology. There is some evidence to support deficits in response inhibition as one such marker. Problems inhibiting prepotent responses are linked with deficits in pre-frontal cortex (PFC) functioning, which is a common dysfunction associated with ADHD and other impulse-control disorders. Evidence-based psychopharmacological and behavioral interventions exist for ADHD. Substance abuse Impulsivity appears to be linked to all stages of substance abuse. The acquisition phase of substance abuse involves the escalation from single use to regular use. Impulsivity may be related to the acquisition of substance abuse because of the potential role that instant gratification provided by the substance may offset the larger future benefits of abstaining from the substance, and because people with impaired inhibitory control may not be able to overcome motivating environmental cues, such as peer pressure. "Similarly, individuals that discount the value of delayed reinforcers begin to abuse alcohol, marijuana, and cigarettes early in life, while also abusing a wider array of illicit drugs compared to those who discounted delayed reinforcers less." Escalation or dysregulation is the next and more severe phase of substance abuse. In this phase individuals "lose control" of their addiction with large levels of drug consumption and binge drug use. Animal studies suggest that individuals with higher levels of impulsivity may be more prone to the escalation stage of substance abuse. Impulsivity is also related to the abstinence, relapse, and treatment stages of substance abuse. People who scored high on the Barratt Impulsivity Scale (BIS) were more likely to stop treatment for cocaine abuse. Additionally, they adhered to treatment for a shorter duration than people that scored low on impulsivity. Also, impulsive