The United Methodist Church has a long history of concern for social justice. Its members have often taken forthright positions on controversial issues involving Christian principles. Early Methodists expressed their opposition to the slave trade, to smuggling, and to the cruel treatment of prisoners.
A social creed was adopted by The Methodist Episcopal Church (North) in 1908. Within the next decade similar statements were adopted by The Methodist Episcopal Church, South, and by The Methodist Protestant Church. The Evangelical United Brethren Church adopted a statement of social principles in 1946 at the time of the uniting of the United Brethren and The Evangelical Church. In 1972, four years after the uniting in 1968 of The Methodist Church and The Evangelical United Brethren Church, the General Conference of The United Methodist Church adopted a new statement of Social Principles, which was revised in 1976 (and by each successive General Conference).
The Social Principles, while not to be considered church law, are a prayerful and thoughtful effort on the part of the General Conference to speak to the human issues in the contemporary world from a sound biblical and theological foundation as historically demonstrated in United Methodist traditions. They are a call to faithfulness and are intended to be instructive and persuasive in the best of the prophetic spirit. The Social Principles are a call to all members of The United Methodist Church to a prayerful, studied dialogue of faith and practice.
You can find the most up to date version of the Social Principles here.
Taken from the preface of the Social Principles at umc.org, http://www.umc.org/what-we-believe/preface