In this paper, they are nicely explicit about their questions and hypotheses:
The objective of this study is twofold. The First, to investigate the association between the self-reported serious and violent delinquency and the 30-bp VNTR in the MAOA gene in a cohort of 2524 adolescents and young adults in the United States in the National Longitudinal Study of Adolescent Health (Add Health). The specific hypothesis is that the MAOA 2R is associated with higher levels of delinquency. Second, to perform a functional analysis that evaluates the promoter activity in an MAOA 1.3-kb promoter-luciferase construct containing 2-, 3-, and 4-repeat sequences of the 30-bp VNTR, using two human brain-derived cell lines: neuroblastoma SH-SY5Y and human glioblastoma 1242-MG....and basic conclusion:
The present study demonstrated a link between the 2 repeat of the 30-bp VNTR in the MAOA gene and much higher levels of self-reported serious and violent delinquency...The VNTR 2 repeat in MAOA and delinquent behavior in adolescence and young adulthood: associations and MAOA promoter activity
Guang Guo, Xiao-Ming Ou, Michael Roettger and Jean C Shih
European Journal of Human Genetics (2008) 16, 626–634
Abstract: Genetic studies of delinquent and criminal behavior are rare in spite of the wide recognition that individuals may differ in their propensity for delinquency and criminality. Using 2524 participants in Add Health in the United States, the present study demonstrates a link between the rare 2 repeat of the 30-bp VNTR in the MAOA gene and much higher levels of self-reported serious and violent delinquency. The evidence is based on a statistical association analysis and a functional analysis of MAOA promoter activity using two human brain-derived cell lines: neuroblastoma SH-SY5Y and human glioblastoma 1242-MG. The association analysis shows that men with a 2R report a level of serious delinquency and violent delinquency in adolescence and young adulthood that were about twice (CI: (0.21, 3.24), P=0.025; and CI: (0.37, 2.5), P=0.008 for serious and violent delinquency, respectively) as high as those for participants with the other variants. The results for women are similar, but weaker. In the functional analysis, the 2 repeat exhibits much lower levels of promoter activity than the 3 or 4 repeat.