The term "missing women" indicates a shortfall in the number of women relative to the expected number of women in a region or country. It is most often measured through male-to-female sex ratios, and is theorized to be caused by sex-selective abortions, female infanticide, and inadequate healthcare and nutrition for female children. It is argued that technologies that enable prenatal sex selection, which have been commercially available since the 1970s, are a large impetus for missing female children. The phenomenon was first noted by the Indian Nobel Prize-winning economist Amartya Sen in an essay in The New York Review of Books in 1990, and expanded upon in his subsequent academic work. Sen originally estimated that more than a hundred million women were "missing" or "gone". Later researchers found differing numbers, with most recent estimates around 90–101 million women. These effects are concentrated in countries typically in Asia (with the largest numbers from India and Mainland China), the Middle East and northern Africa. Economists such as Nancy Qian and Seema Jayanchandran have found that a large part of the deficit in China and India is due to lower female wages and sex-selective abortion, or differential neglect. However, the disparity has also been found in Chinese and Indian immigrant communities in the United States, albeit to a far lesser degree than in Asia. An estimated 2000 Chinese and Indian female fetuses were aborted between 1991 and 2004, and a shortage can be traced back as far as 1980. Some countries in the former Soviet Union also saw declines in female births after the revolutions of 1989, particularly in the Caucasus region. Also the Western world saw a dramatic drop in female births since the 1980s. Researchers have also argued that other diseases, HIV/AIDS, natural causes, and female abduction are also responsible for missing women. However, son preference, as well as associated reasons to care for male well-being over female well-being, is still considered to the primary cause. In addition to the health and wellbeing of women, the missing women phenomenon has led to an excess of males in society and an imperfectly balanced marriage market. Because of the association of missing women with female neglect, countries with higher rates of missing women also tend to have higher rates of women in poor health, leading to higher rates of infants in poor health. Researchers argue that increasing women's education and women's employment opportunities