Parent management training was developed in the early 1960s in response to an urgent need in American child guidance clinics. Research across a national network of these clinics revealed that the treatments being used for young children with disruptive behaviors, who constituted the majority of children served in these settings, were largely ineffective. Several child psychologists, including Robert Wahler, Constance Hanf, Martha E. Bernal, and Gerald Patterson, were inspired to develop new treatments based on behavioral principles of operant conditioning and applied behavioral analysis. Between 1965 and 1975, a behavioral model of parent training treatment was established, that emphasized teaching parents to positively reinforce prosocial child behavior (such as praising a child for following directions) while negatively incentivizing antisocial behavior (such as removing parental attention after the child throws a tantrum).: 169–170  The early work of Hanf and Patterson hypothesized that "teaching parents the principles of behavioral reinforcement would result in effective, sustainable change in child behavior".  Early studies of this approach showed that the treatment was effective in the short-term in improving parenting skills and reducing children's disruptive behaviors. Patterson and colleagues theorized that adverse environmental contexts lead to disruptions in parent practices, which then contribute to negative child outcomes.: 161 Following the initial development of PMT, a second wave of research from 1975 to 1985 focused on the longer-term effects and generalization of treatment to settings other than the clinic (such as home or school), larger family effects (such as improved parenting with siblings), and behavioral improvements outside of the targeted areas (such as improved ability to make friends). Since 1985, the literature on PMT has continued to expand, with researchers exploring such topics as application of the treatment to serious clinical problems, dealing with client resistance to treatment, prevention programs, and implementation with diverse populations.: 170–174 Evidence in support of PMT has not always been rigorously examined;  future research should examine the effectiveness of PMT on the families most at risk, address parental psychopathology as a factor in outcomes, examine whether gains from PMT are maintained in the long-term, and better account for variability in outcomes dependent on practices under "real-world" conditions.