The interactions of genetic and environmental sources of variation produce powerful individual differences in the development of neural mechanisms that are permissive for a variety of aggressive behaviors. Ultimately, profound dif­ ferences in these behaviors are therefore observable both between species and within species. Numerous genes are essential for the normal development and function of the neural systems mediating aggressive behaviors. Detection of the variants of these genes will not reduce behavior to the nucleotide, but could help explain the origins of behavior and the origins of behavioral variation. The development and eventual function of the neural systems involved in aggression are powerfully modulated in many ways by the animal’s experiences, and the animal’s experiences are shaped by gene-environment interactions. The domain of this chapter is the detection of gene variants which, in part, mediate the interindividual variation.