Data support intervening on the neighborhood level, as well, to prevent multiple problems. The Moving to Opportunities program (MTO), sponsored by the U.S. Department of Housing and Urban Development (HUD) involves giving vouchers to experimental group families living in low-income neighborhoods that can be used only in higher income areas. This project used a randomized experimental design by issuing the vouchers through a lottery. The experimental group also received counseling and assistance in finding a private rental unit. A comparison group received section 8 vouchers that could be used anywhere and a control group continued to receive current project-based assistance. It should be noted, though, that all of the participants in MTO volunteered. Therefore, caution should be taken when generalizing the results. One early paper on the impacts of this program suggested significant, positive effects of moving to a higher income neighborhood, such as improved child and parent mental health (e.g., depression and anxiety) as well as lower rates of youth delinquency and problem behaviors (Del Conte & Kling, 2001). The authors point to Jencks and Mayer's (1990) theory that there is a contagion effect in which participation in certain activities increases with the proportion of peers who engage in that behavior. Therefore, a negative social climate should result in more problem behaviors, while a more positive social climate (e.g., lower crime) should result in fewer problem behaviors. Also, having an increase in human capital may provide children and adolescents with more positive support (Borjas, 1995). In Boston, the experimental group boys (8-14 years old) were found to have 27% fewer behavior problems (e.g., disobedience at home, bullying others, inability to sit still, depression) than the control group. The authors referred to the fact that girls reduced their social contact in the new neighborhoods by 30% as a reason for no significant decline in female problem behaviors. In New York, the experimental group boys (8-18 years old) were happier, less depressed, and had fewer instances of arguing than the control group.