By conferring allele-specific transcriptional activity on the monoamine oxidase 
A (MAOA) gene in humans, length variation of a repetitive sequence [(variable 
number of tandem repeat (VNTR)] in the MAOA promoter influences a constellation 
of personality traits related to aggressive and antisocial behavior and 
increases the risk of neurodevelopmental and psychiatric disorders. Here, we 
have analyzed the presence and variability of this MAOA promoter repeat in 
several species of nonhuman primates. Sequence analysis of MAOA's 
transcriptional control region revealed the presence of the VNTR in chimpanzee 
(Pan troglodytes), bonobo (Pan paniscus), gorilla (Gorilla gorilla), orangutan 
(Pongo pygmaeus), rhesus macaque (Macaca mulatta) and Gelada baboon 
(Theropithecus gelada). The majority of P. troglodytes and P. paniscus showed a 
single repeat with a sequence identical to the VNTR sequence in humans. In 
contrast, analyses of the remaining species revealed shorter sequences similar 
to the first 18 bp of human VNTR. Compared with other nonhuman primates, the 
VNTR sequence of M. mulatta showed the highest length variability with allele 
frequencies of 35, 25 and 40% for the five, six and seven repeat variants, 
respectively. The extent of variability of the MAOA promoter repeat in both 
rhesus monkeys and humans supports the notion that there may be a relationship 
between functional MAOA expression and aggression-related traits in humans and 
rhesus macaque populations.