Twin and adoption studies indicate that both conduct disorder and oppositional defiant disorder are heritable.
For example, one study of 1,116 pairs of 5-year-old twins found that 82 percent of the variability in conduct disorder was due to genetic factors.
Genetics appears to play a particularly strong role in conduct disorder that begins in early childhood.
Several specific genes have been associated with an increased risk of conduct disorder and oppositional defiant conduct, primarily genes involved in the regulation of the neurotransmitters dopamine, serotonin, and norepinephrine.
One study found an interaction between childhood maltreatment (including physical and sexual abuse as well as neglect) and the MAOA gene, which encodes an enzyme that metabolizes dopamine, serotonin, and norepinephrine.
Children who both were maltreated and had the abnormal variant of the MAOA gene were especially likely to develop conduct disorder.