The Pope said his letter would look at modern problems in the field of promoting development, and he asked for prayers for "this latest contribution that the church offers humanity in its commitment for sustainable progress in full respect for human dignity and the real needs of all."

Instead of focusing on theological beliefs, the social encyclicals written by most modern-day popes have tried to shape the way Christians and all people of good will can better serve the common good. Each social encyclical was unique in that it sought to respond to the most pressing social realities at the time.

Radically new problems caused by the modern industrial age prompted Pope Leo XIII to issue the church's groundbreaking social encyclical in 1891. It was the first time the church spoke in a comprehensive and official way on social concerns, and it ushered in the era of Catholic social teaching.

The document, Rerum Novarum (on capital and labour), highlighted the condition of the working class and insisted that development must include social progress as well as economic growth. Pope Leo defended the right of workers to organize to seek higher wages and better working conditions; detailed the rights and obligations of management and labour; and opposed the Marxist concept of abolishing private property.

Pope Pius XI's social encyclical Quadragesimo Anno (on reconstructing the social order) came out in 1931, the 40th anniversary of Pope Leo's encyclical. The Great Depression was in full swing at the time, causing many to question the benefit of the reigning capitalist and communist economic systems.

Pope Pius insisted that true socialism is "utterly foreign to Christian truth" since its concept of life is material rather than spiritual. Yet he also warned that unbridled capitalism was producing "economic imperialism" by concentrating wealth and economic power in the hands of a few.

On the 70th anniversary of Rerum Novarum, Pope John XXIII issued Mater et Magistra (Mother and Teacher), which described the church as mother and teacher on social issues. Dedicated to Christianity and social progress, the 1961 letter said the duty to bring social justice to the world was not the responsibility of individuals alone, but that the state shared that obligation.

Pope John held that "fruitful and lasting" peace is impossible if the gap between people's living conditions is too great, and he called for broad international co-operation to help underdeveloped nations overcome their "permanent state of poverty, of misery or of hunger."

Pope John's second social encyclical, Peace on Earth (Pacem in Terris), was issued in 1963 at the height of the Cold War. Echoing a theme in his first social document, he underlined the necessity of having adequate, effective international structures to help nations move toward greater justice and peace in an increasingly interdependent world.

In 1967, Pope Paul VI wrote his first and only social encyclical. It was a time when the world was starkly divided into two political blocs, East and West. Cold War tensions were high, and wars were raging in the Middle East and in Vietnam.

However, in Populorum Progressio (The Progress of Peoples), Pope Paul focused not on the U.S.-Soviet faceoff but on the world's peoples, who had become starkly divided between those who enjoyed a high standard of living and those who struggled with poverty and underdevelopment.

Authentic development is the key to achieving real peace, and it must include the development of all people and the whole person, both materially and in their relationship with God, he said.

Elected in 1978, Pope John Paul II made repeated appeals throughout his pontificate for social and economic justice and warned about the dangers of globalization. His social teaching was distilled in three major encyclicals. The first, On Human Work (Laborem Exercens), was issued in 1981 and criticized the abuses of a "rigid capitalism," which placed profit above the well-being of workers. But, having lived in communist Poland, Pope John Paul also said Marxism's class struggle was not the answer.

His second social encyclical, On Social Concerns (Sollicitudo Rei Socialis), was published in 1987, the 20th anniversary of Pope Paul's Populorum Progressio. Again, the pope was sharply critical of communism and unbridled capitalism. He warned of the ever-widening gap between rich and poor countries and cited the crushing foreign debt of developing nations as a major contributor to the problem.

The encyclical was considered to be a breakthrough document on ecology as well because of its tough language on the need to protect the environment. The pope said the dominion granted humans over the natural world has biological and moral limits that cannot be violated in the name of development.

In 1991, the 100th anniversary of Rerum Novarum, Pope John Paul issued his third document on social issues, Centesimus Annus (The Hundredth Year). It analysed the social situation in the light of communism's collapse and called for reform of the free-market system. While important and valuable for a prosperous economy, the free market could not address all fundamental human needs, and it must be set in an ethical and legal framework, Pope John Paul said.

His 1995 encyclical, Evangelium Vitae (The Gospel of Life), which addressed the sacredness of all human life, is considered a social encyclical by many people because it included strong statements on the need for the political world to do its part in protecting human life. The encyclical rejected the argument that Catholic politicians could separate their private consciences from public conduct. And it insisted that laws allowing abortion and euthanasia are not morally binding and require "conscientious objection" by the faithful.

In 2004, the Pontifical Council for Justice and Peace published the Compendium of the Social Doctrine of the Church, providing a concise and complete overview of the church's social teaching. Covering everything from work to the family and from politics to the environment, the compendium showed how church teaching and pastoral action have developed over time.

Anticipating his first social encyclical, Pope Benedict said it would offer "a beautiful response" to the new realities and changes that had occurred since Centesimus Annus was promulgated 18 years ago.

Pope Benedict also said the publication of the document was delayed by the eruption of one of the worst global economic crises in decades. He said he wanted to update what he had drafted so the document would deal thoroughly with the current crisis and offer "a more adequate response" to the world's financial woes.