Some of the wealthiest nations in the world, including the United States, drop out of top 20 in the 2011 Human Development Index (HDI) when the index is adjusted for internal inequalities, said a report on Wednesday. According to the Human Development Report 2011 issued by the United Nations Development Program (UNDP) here on Wednesday, Norway, Australia and the Netherlands lead the world in the HDI. The United States, New Zealand, Canada, Ireland, Liechtenstein, Germany and Sweden are also among the top 10 countries in HDI, but the United States falls from No. 4 to No. 23, South Korea from No. 15 to No. 32 and Israel from No. 17 to No. 25 when the index is adjusted for internal inequalities in health, education and income. The drop of the United States and Israel is mainly because of income inequality, said the report presented by Veerle Vandeweerd, Director of Environment and Energy Group in Development Policy Bureau of the UNDP. \"The theme for this year\'s report is Sustainability and Equity, A Better Future for All,\" said Vandeweerd. There has been progress globally in schooling, but inequality problem is getting serious in the world, she added. Environmental degradation can be seen in terms of access to modern cooking fuel, clean water and basic sanitation, Vandeweerd said. Therefore, to eradicate poverty and inequality, equal access to climate financing and to affordable and clean energy is the key, she said. Many countries have made progress in average HDI level. Over 40 percent of the countries in the world have made great progress in health, education and income. Sweden ranks No. 1 in gender equality in reproductive health, expected years of schooling as well as women\'s representation in the parliament and in the job market. Japan has the highest life expectancy of 83.4 years old, while Australia has the longest schooling years of 18 years.