Neurochemical and pharmacological findings
The primary pharmacological treatments for ADHD are methylphenidate (Ritalin) and amphetamine. Both methylphenidate and amphetamines block re-uptake of dopamine and norepinephrine into the pre-synaptic neuron, acting to increase post-synaptic levels of dopamine and norepinephrine. Of these two monoamines, increased availability of dopamine is considered the primary cause for the ameliorative effects of ADHD medications, whereas increased levels of norepinephrine may be efficacious only to the extent that it has downstream, indirect effects on dopamine. The effectiveness of dopamine re-uptake inhibitors in treating the symptoms of ADHD has led to the hypothesis that ADHD may arise from low tonic levels of dopamine (particularly in the fronto-limbic circuitry), but evidence in support of this theory is mixed. Genetics
There are several difficulties when it comes to trying to identify a gene for complex traits such as impulsivity, such as genetic heterogeneity. Another difficulty is that the genes in question might sometimes show incomplete penetrance, "where a given gene variant does not always cause the phenotype". Much of the research on the genetics of impulsivity-related disorders, such as ADHD, is based on family or linkage studies. There are several genes of interest that have been studied in an attempt to find the major genetic contributors to impulsivity. Some of these genes are:

DAT1  is the dopamine transporter gene which is responsible for the active reuptake of dopamine from the neural synapse. DAT1 polymorphisms have been shown to be linked to hyperactivity and ADHD.
ICD NOS includes other significant difficulties that seem to be related to impulsivity but do not meet the criteria for a specific DSM diagnosis.There has been much debate over whether or not the ICDs deserve a diagnostic category of their own, or whether they are in fact phenomenologically and epidemiologically related to other major psychiatric conditions like obsessive-compulsive disorder (OCD), affective disorders, and addictive disorders. In fact, the ICD classification is likely to change with the release of the DSM-V in May 2013. In this new revision the ICD NOS will likely be reduced or removed; proposed revisions include reclassifying trichotillomania (to be renamed hair-pulling disorder) and skin-picking disorder as obsessive-compulsive and related disorders, moving intermittent explosive disorder under the diagnostic heading of disruptive, impulse control, and conduct disorders, and gambling disorder may be included in addiction and related disorders.The role of impulsivity in the ICDs varies. Research on kleptomania and pyromania is lacking, though there is some evidence that greater kleptomania severity is tied to poor executive functioning.Trichotillomania and skin-picking disorder seem to be disorders that primarily involve motor impulsivity, and will likely be classified in the DSM-V within the obsessive-compulsive and related disorders category.Pathological gambling, in contrast, seems to involve many diverse aspects of impulsivity and abnormal reward circuitry  (similar to substance use disorders) that has led to it being increasingly conceptualized as a non-substance or behavioral addiction.