In 1970 he won a fellowship from the United Church Board for World Ministries to study US juvenile criminal justice at Boston University.
[4] In 1976, after having read his master's thesis, Dean Irene Cortes of the University of the Philippines College of Law invited Tadiar to join the faculty.
For 12 years (1980–1992), he was a member of the Committee of Consultants, Bureau of Local Government Supervision, which oversaw the nationwide operations of the Katarungang Pambarangay Law.
[citation needed] A significant finding of his study was that Filipino people placed a high value on the legal recognition of their customary mode of settling their interpersonal disputes for its own intrinsic worth, while the instrumental value of decongesting court dockets is only of secondary importance.
The result of his study entitled "Court-Referred Mediation" bore legal fruit in 1999 by an official pilot project for such diversion in the cities of Valenzuela and Mandaluyong.
Tadiar's bent for law reform, particularly in the judiciary, resulted in his appointment by the Supreme Court as member of the Design and Management Committee of the ADR Model Courts for the two pilot projects (Justice Reforms Initiatives Support Project funded by the Canadian International Development Agency) [9] in San Fernando, Pampanga and Bacolod, Negros Occidental.
He served as the first Executive Director of the pioneering Center for Research, Studies and Training on Reproductive Health, Rights and Ethics (REPROCEN) as a joint project of the UP Colleges of Law and Medicine.
From the 1980s, he served at various times, as Chairman or Member of the Board of Directors of two other Philippine NGOs—the Women's Health Care Foundation and Institute for Social Studies and Action.