In some respects the biggest change was the concept that sovereinty rested with the people of Japan, not with the Emperor. Of course, the Emperor did not really have anything to do with the actual running of the government except by indirect influence, but it was still a radical idea to the Japanese. Many older citizens refused to accept this, but two generations have now grown up with the Emperor as a figurehead more than the sacred head of state. The Diet, or assembly of elected representatives, became the real government, replacing the military which had been the real source of power for generations.
Women were given equal rights and the vote, and other rights such as free speech were guarenteed to the people. Most importantly the military was temporarily disbanded, and reconstituted without the political power they had wielded before.
Economically, land reform gave ownership of land to the farmers (50% of the work force) instead of the large landlords who took up to half the value of crops in rent every year, which allowed the farmers more independence and a greater share of the nation's economy. Free trade unions were also legalized.
The Occupation Authorities used control of the newspapers and radio to educate the populace about American ideas of democracy, and changed the educational system to teach these ideals. The laws regarding the head of household's total control over the family were also changed. These were the major social changes of the late 1940s and early '50s in Japan.
Under the leadership of General Douglas McArthur, the Allied administration, known as SCAP (Supreme Commander for the Alied Powers) implemented policies that aimed to demilitarise and democraticise Japan. In 1947, they wrote a new constitution for Japan, which included the highly controversial Article 9 that saw Japan renounce war forever as a sovereign nation. Ideas of democracy were promoted and Japanese women were granted the right to vote. After 1947, Cold War politics increasingly dominated American interests and their subsequent policies in Japan - they became more willing to sacrifice democracy for Cold War purposes. American policy-makers sought to use Japan as a linchpin for their Cold War policy in the Asia Pacific and they attempted to do so through economic reconstruction by creating social stability in the country. This prevented communists in Japan from having a decisive cause to rally the people around. Japan was also compelled to sign the San Francisco Peace Treaty in 1951 as one of the conditions for independence. They were also drawn into forming a mutual defence alliance with the US, as witnessed in the signing of the US-Japan Mutual Security Treaty in 1951, which allowed American troops and military bases to remain in Japan even after the nation had regained sovereignty.