The 10/40 Window is a term coined by Christian missionary strategist and Partners International CEO Luis Bush in 1990 to refer to those regions of the eastern hemisphere, plus the European and African part of the western hemisphere, located between 10 and 40 degrees north of the equator, a general area that was purported to have the highest level of socioeconomic challenges and least access to the Christian message and Christian resources on the planet.
The concept behind the 10/40 Window highlights these three elements (as of data available in 1990): an area of the world with great poverty and low quality of life, combined with lack of access to Christian resources. The Window forms a band encompassing Saharan and Northern Africa, as well as almost all of Asia (West Asia, Central Asia, South Asia, East Asia and much of Southeast Asia). Roughly two-thirds of the world population lived in the 10/40 Window, and it is predominantly Muslim, Hindu, Buddhist, animist, Jewish, or atheist. Many governments in the 10/40 Window are officially or unofficially opposed to Christian work of any kind within their borders.
This region of the world was previously known to Christians as the "resistant belt", as noted by Luis Bush at the 1989 Lausanne II Conference in Manila. In 1990, Bush's research led to a meeting with Pete Holzmann, a leader of the team developing the first PC-based GIS software. They analyzed the region (see below) using a box between 10 and 40 degrees north latitude and called it the 10/40 box. A few weeks later, Bush and his wife Doris were inspired to rename it the 10/40 Window, stating that this region ought to be seen as a "window of opportunity". The analysis and concept was a generalization that focuses on a region, not a sharp boundary defining what is a priority, and what is not. For this reason, many missiologists prefer to use the phrase 10/40 Window region.
Before being called the "resistant belt", the Islamic portions of this region, as well as selected unreached Buddhist and Hindu areas, were referred to as the "unoccupied fields" by Samuel Zwemer, in his book by that same title, published in 1911.
Some have objected to such a broad-brush term which seems to imply a unifying characteristic of the 10/40 Window when in fact no large area of the planet is completely homogenous in cultural attributes.
The 1990 research data states:
- this part of the world was home to the largest populations living in deep poverty and lowest quality of life;
- this part of the world did have least access to Christian resources. Note the emphasis on access not percent Christian. The West has ubiquitous access to such resources; this area of the world did and does not.
This research deals in overall population characteristics. The 10/40 Window is a term that helps people visualize the general area of the analysis, where the above characteristics are generally true, but with exceptions proving it is only a generalization. Some examples of the exceptions:
- The 10/40 Window article refers to the "poorest of the poor" living in that region (based on late-1980s per-capita GNP under US$500). Of the three billion people living in such poverty-stricken nations, 82% lived in the 10/40 Window. Yet the 10/40 Window also includes nations such as South Korea and Japan. In the late 20th century, Japan boasted the world's third largest economy, and South Korea the eleventh. Such nations were strong throughout the late 20th century.
- Geographically, the 10/40 Window includes the Philippines and Portugal, which both have a Roman Catholic Christian majority; Greece, where almost 98% of the population belonged to the Greek Orthodox Church; and South Korea, which is home to the largest single congregation church in the world and is also the second largest missionary-sending nation in the world. The 10/40 Window does not encompass Indonesia or Sri Lanka, nations that had very little access to Christian resources.
Gaining widespread use
Over the years, the 10/40 Window has evolved from a specialist term used by Christian missiologists to assumed vocabulary for Christians in the West. It is an emerging term in the secular press and can be found in press style glossaries. Non-western writers and organizations also refer to the 10/40 Window. In addition, those opposed to the idea of evangelism make use of the term.  
The original 1990 GIS 10/40 Window analysis produced several insights, among them showing that the nations of the 10/40 Window represented (as of the research date):
- 82% of the poorest of the world's poor (per capita GNP less than US$500 per year),
- 84% of those with lowest quality of life (life expectancy, infant mortality, and literacy),
- the hub of the world's major non-Christian religions (Islam, Buddhism, Hinduism, etc.,)
- close to 100% of those who were both most-poor and had least-access to Christian resources (two-dimensional analysis)
- The least Christian resource investment and least sharing of the Christian message
Non-Christians in the 10/40 Window by religion
The first edition GIS analysis maps highlighted the three major religious blocks in the 10/40 Window, specifically the majority Muslim, Hindu and Buddhist nations. Population estimates at the time for the year 2000 (from Operation World) were given as:
- 28 Muslim countries, 1.1 billion population est. (2000)
- 2 Hindu countries, 1.1 billion population est. (2000)
- 8 Buddhist countries, 237 million population est. (2000)
Later updates have been based more on census data and other estimates rather than forward-looking population estimates. The cited reference provides the following estimate of "unreached" non-Christian populations in the 10/40 Window:
- 865 million Muslims
- 550 million Hindus
- 275 million Buddhists
- 140 million in 2550 tribal groups (mainly animist)
- 17 million Jews
Nations in the 10/40 Window
- Burkina Faso
- Israel (including Palestinian Occupied Territory as of 1998)
- Korea, North
- Korea, South
- Saudi Arabia
- Sudan (includes South Sudan as of 2011, due to Sudan splitting into two nations)
- United Arab Emirates
- Western Sahara
These were all of the "Old World" nations (mostly in the eastern hemisphere) with at least 50 percent of their land mass falling within 10 to 40 degrees latitude as of 1990. (The list also included Macau and Gibraltar, which are not independent nations.) See the list of nations in the original study.
- Child evangelism movement
- Human Development Index which followed from the Quality of Life Index used in the 10/40 Window research
- "The 10/40 Window". Time. June 30, 2003. Archived from the original on April 28, 2007. Retrieved March 1, 2007.. The link is to the map, which is part of an extensive cover story.
- World FactBook., edition available in 1990. At the time, the authoritative source for socioeconomic and political metrics for every nation. Extreme poverty was denoted as under US$500 per capita GNP (in 1990 dollars). Human suffering was measured by the Quality of Life index, precursor to today's Human Development Index
- See the Analysis section of this article for research-based details and cited references.
- "Access" is generally defined using a variety of metrics. What is least controversial is those areas with least access throughout history, as all metrics for such areas are zero or close to zero. Examples of metrics used include the presence of (Christian) work and workers (of any kind, whether community development, health, business, child care, house servants, etc), media in an appropriate language (print, TV, radio, web, etc.),
- Operation World provides a concise well-researched/cited data summary for each nation
- Barrett, David B.; Kurian, George T.; Johnson, Todd M. (eds.) (2001). World Christian Encyclopedia (2nd ed.). Oxford University Press. pp. 1739pp. ISBN 0-19-507963-9. Archived from the original on February 6, 2003. Much of its data is available online at the ("World Christian Database". Brill. Archived from the original on March 4, 2007. Retrieved March 1, 2007.) Mind-numbing in its details (with some areas of unique value), but the introduction and definitions in the paper edition are quite helpful to understanding.
- Atlas GIS, produced by Strategic Mapping Inc., was the first PC Geographic Information System."ESRI Retires ArcCAD and Atlas GIS". Archived from the original on September 25, 2007. Retrieved September 21, 2007.
- Zwemer, Samuel M. (1911). The Unoccupied Mission Fields of Africa and Asia. New York: Student Volunteer Movement for Foreign Missions. p. 260.
- Verbal and online discussions, no documentation available outside of Wikipedia itself.
- The Poorest Countries & The 10/40 Window (Map) (August 1, 1990 ed.). Cartography by GMI/GRDB. AD2000.
- Quality of Life & The 10/40 Window (Map) (August 1, 1990 ed.). Cartography by GMI/GRDB. AD2000.
- The largest congregation is in Seoul (Yoido Full Gospel Church) "The Top 10 Churches in the World (by size)". Archived from the original on September 27, 2007. Retrieved August 20, 2007.
- "Missions Incredible". Archived from the original on November 7, 2006.
- "Becoming Global Christians in the 21st Century By Justin Long – Charisma Magazine, (2001)". Charismamag.com. Retrieved November 20, 2011.
- "Islam and China's Christmas By David Aikman – Charisma Magazine, (2002)". Charismamag.com. Retrieved November 20, 2011.
- "Marli Spieker | Today's Christian Woman | Your Life. Your Faith. Your World, (2003)". Christianitytoday.com. Retrieved November 20, 2011.
- "The Faith at Work Movement: Opening "The 9 to 5 Window" – FaithInTheWorkplace.com, (2004)". Christianitytoday.com. October 30, 2011. Retrieved November 20, 2011.
- "Evangelicals look to '10/40 Window' | National Catholic Reporter Conversation Cafe, (2006)". Archived from the original on 2007-09-27. Retrieved November 20, 2011.
- TIME: The 10/40 Window
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- Three Religious Blocks & The 10/40 Window (Map) (August 1, 1990 ed.). Cartography by GMI/GRDB. AD2000.
- Islam & & The 10/40 Window (Map) (August 1, 1990 ed.). Cartography by GMI/GRDB. AD2000.
- The Poor, The Unevangelized, & The 10/40 Window (Map) (August 1, 1990 ed.). Cartography by GMI/GRDB. AD2000.
- Well below 10 percent in any of the study populations. Each of the cited maps provides side data on population, Christian involvement, etc in the various study populations (poor, low quality of life, non-Christian, etc)
- The 55 Least Evangelized Countries & The 10/40 Window (Map) (August 1, 1990 ed.). Cartography by GMI/GRDB. AD2000.
- Note that this number is higher than the census-based world total of 14 million. However, even authoritative Jewish sources state that many Jews do not identify themselves in population censuses.
- "What is the 10/40 Window?". joshuaproject.net. Retrieved May 21, 2018.
- Long, Justin (January 2001). "Islam and China's Christmas". Charisma Magazine. Retrieved March 1, 2007.
- Aikman, David (January 2002). "Becoming Global Christians in the 21st century". Charisma Magazine. Retrieved March 1, 2007.
- Walker, J Gary (April 2003). "Marli Spieker, Broadcasting Hope". Christianity Today. Archived from the original on February 15, 2007. Retrieved March 1, 2007.
- Hillman, Os (2004). "The Faith At Work Movement". Christianity Today. Archived from the original on February 19, 2007. Retrieved March 1, 2007.
- Allen, John L (September 2006). "Evangelicals Look To 10/40 Window". National Catholic Reporter. Retrieved March 1, 2007.
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- Updated statistics, including some relating to 10/40 Window
- 10/40 Window Maps Updated data from the original GIS analysis